diff --git a/src/background/index.js b/src/background/index.js
index 3508e28..cb925f3 100644
--- a/src/background/index.js
+++ b/src/background/index.js
@@ -2,12 +2,15 @@
 // Handles EIP-1193 RPC requests from content scripts and proxies
 // non-sensitive calls to the configured Ethereum JSON-RPC endpoint.
 
-const {
-    ETHEREUM_MAINNET_CHAIN_ID,
-    DEFAULT_RPC_URL,
-} = require("../shared/constants");
+const { DEFAULT_RPC_URL } = require("../shared/constants");
+const { SUPPORTED_CHAIN_IDS, networkByChainId } = require("../shared/networks");
 const { getBytes } = require("ethers");
-const { state, loadState, saveState } = require("../shared/state");
+const {
+    state,
+    loadState,
+    saveState,
+    currentNetwork,
+} = require("../shared/state");
 const { refreshBalances, getProvider } = require("../shared/balances");
 const { debugFetch } = require("../shared/log");
 const { decryptWithPassword } = require("../shared/vault");
@@ -329,31 +332,47 @@ async function handleRpc(method, params, origin) {
     }
 
     if (method === "eth_chainId") {
-        return { result: ETHEREUM_MAINNET_CHAIN_ID };
+        return { result: currentNetwork().chainId };
     }
 
     if (method === "net_version") {
-        return { result: "1" };
+        return { result: currentNetwork().networkVersion };
     }
 
     if (method === "wallet_switchEthereumChain") {
         const chainId = params?.[0]?.chainId;
-        if (chainId === ETHEREUM_MAINNET_CHAIN_ID) {
+        if (chainId === currentNetwork().chainId) {
+            return { result: null };
+        }
+        if (SUPPORTED_CHAIN_IDS.has(chainId)) {
+            // Switch to the requested network
+            const target = networkByChainId(chainId);
+            state.networkId = target.id;
+            state.rpcUrl = target.defaultRpcUrl;
+            state.blockscoutUrl = target.defaultBlockscoutUrl;
+            await saveState();
+            broadcastChainChanged(target.chainId);
             return { result: null };
         }
         return {
             error: {
                 code: 4902,
-                message: "AutistMask only supports Ethereum mainnet.",
+                message:
+                    "AutistMask supports Ethereum Mainnet and Sepolia Testnet only.",
             },
         };
     }
 
     if (method === "wallet_addEthereumChain") {
+        const chainId = params?.[0]?.chainId;
+        if (SUPPORTED_CHAIN_IDS.has(chainId)) {
+            return { result: null };
+        }
         return {
             error: {
                 code: 4902,
-                message: "AutistMask only supports Ethereum mainnet.",
+                message:
+                    "AutistMask supports Ethereum Mainnet and Sepolia Testnet only.",
             },
         };
     }
@@ -499,6 +518,27 @@ async function handleRpc(method, params, origin) {
     return { error: { message: "Unsupported method: " + method } };
 }
 
+// Broadcast chainChanged to all tabs when the network is switched.
+function broadcastChainChanged(chainId) {
+    tabsApi.query({}, (tabs) => {
+        for (const tab of tabs) {
+            tabsApi.sendMessage(
+                tab.id,
+                {
+                    type: "AUTISTMASK_EVENT",
+                    eventName: "chainChanged",
+                    data: chainId,
+                },
+                () => {
+                    if (runtime.lastError) {
+                        // expected for tabs without our content script
+                    }
+                },
+            );
+        }
+    });
+}
+
 // Broadcast accountsChanged to all tabs, respecting per-address permissions
 async function broadcastAccountsChanged() {
     // Clear non-remembered approvals on address switch
diff --git a/src/content/inpage.js b/src/content/inpage.js
index 9a95012..8147f49 100644
--- a/src/content/inpage.js
+++ b/src/content/inpage.js
@@ -2,7 +2,10 @@
 // Creates window.ethereum (EIP-1193 provider) and announces via EIP-6963.
 
 (function () {
-    const CHAIN_ID = "0x1"; // Ethereum mainnet
+    // Defaults to mainnet; updated dynamically via eth_chainId on init and
+    // chainChanged events from the extension.
+    let currentChainId = "0x1";
+    let currentNetworkVersion = "1";
 
     const listeners = {};
     let nextId = 1;
@@ -28,6 +31,12 @@
         if (event.source !== window) return;
         if (event.data?.type !== "AUTISTMASK_EVENT") return;
         const { eventName, data } = event.data;
+        if (eventName === "chainChanged") {
+            currentChainId = data;
+            currentNetworkVersion = String(parseInt(data, 16));
+            provider.chainId = currentChainId;
+            provider.networkVersion = currentNetworkVersion;
+        }
         emit(eventName, data);
     });
 
@@ -57,8 +66,8 @@
     const provider = {
         isAutistMask: true,
         isMetaMask: true, // compatibility — many dApps check this
-        chainId: CHAIN_ID,
-        networkVersion: "1",
+        chainId: currentChainId,
+        networkVersion: currentNetworkVersion,
         selectedAddress: null,
 
         async request(args) {
@@ -75,6 +84,12 @@
                         ? result[0]
                         : null;
             }
+            if (args.method === "eth_chainId" && result) {
+                currentChainId = result;
+                currentNetworkVersion = String(parseInt(result, 16));
+                provider.chainId = currentChainId;
+                provider.networkVersion = currentNetworkVersion;
+            }
             return result;
         },
 
@@ -189,4 +204,19 @@
 
     window.addEventListener("eip6963:requestProvider", announceProvider);
     announceProvider();
+
+    // Fetch the current chain ID from the extension on load so the provider
+    // reflects the selected network immediately (covers Sepolia etc.).
+    sendRequest({ method: "eth_chainId", params: [] })
+        .then((chainId) => {
+            if (chainId) {
+                currentChainId = chainId;
+                currentNetworkVersion = String(parseInt(chainId, 16));
+                provider.chainId = currentChainId;
+                provider.networkVersion = currentNetworkVersion;
+            }
+        })
+        .catch(() => {
+            // Best-effort — keep defaults.
+        });
 })();
diff --git a/src/popup/index.html b/src/popup/index.html
index 821571d..90fb615 100644
--- a/src/popup/index.html
+++ b/src/popup/index.html
@@ -882,6 +882,24 @@
                     </div>
                 </div>
 
+                <div class="bg-well p-3 mx-1 mb-3">
+                    <h3 class="font-bold mb-1">Network</h3>
+                    <p class="text-xs text-muted mb-1">
+                        Select the Ethereum network. Switching networks will
+                        update the RPC and Blockscout endpoints to their
+                        defaults.
+                    </p>
+                    <div class="text-xs flex items-center gap-1">
+                        <select
+                            id="settings-network"
+                            class="border border-border p-1 bg-bg text-fg text-xs cursor-pointer"
+                        >
+                            <option value="mainnet">Ethereum Mainnet</option>
+                            <option value="sepolia">Sepolia Testnet</option>
+                        </select>
+                    </div>
+                </div>
+
                 <div class="bg-well p-3 mx-1 mb-3">
                     <h3 class="font-bold mb-1">Ethereum RPC</h3>
                     <p class="text-xs text-muted mb-1">
diff --git a/src/popup/views/addressDetail.js b/src/popup/views/addressDetail.js
index 3ef4951..169ab10 100644
--- a/src/popup/views/addressDetail.js
+++ b/src/popup/views/addressDetail.js
@@ -9,7 +9,12 @@ const {
     escapeHtml,
     truncateMiddle,
 } = require("./helpers");
-const { state, currentAddress, saveState } = require("../../shared/state");
+const {
+    state,
+    currentAddress,
+    saveState,
+    currentNetwork,
+} = require("../../shared/state");
 const { formatUsd, getAddressValueUsd } = require("../../shared/prices");
 const {
     fetchRecentTransactions,
@@ -36,7 +41,7 @@ const EXT_ICON =
     `</svg></span>`;
 
 function etherscanAddressLink(address) {
-    return `https://etherscan.io/address/${address}`;
+    return `${currentNetwork().explorerUrl}/address/${address}`;
 }
 
 function show() {
diff --git a/src/popup/views/addressToken.js b/src/popup/views/addressToken.js
index 78b03c9..8b31b60 100644
--- a/src/popup/views/addressToken.js
+++ b/src/popup/views/addressToken.js
@@ -12,7 +12,12 @@ const {
     truncateMiddle,
     balanceLine,
 } = require("./helpers");
-const { state, currentAddress, saveState } = require("../../shared/state");
+const {
+    state,
+    currentAddress,
+    saveState,
+    currentNetwork,
+} = require("../../shared/state");
 const { TOKEN_BY_ADDRESS, resolveSymbol } = require("../../shared/tokenList");
 const {
     formatUsd,
@@ -42,7 +47,7 @@ const EXT_ICON =
     `</svg></span>`;
 
 function etherscanAddressLink(address) {
-    return `https://etherscan.io/address/${address}`;
+    return `${currentNetwork().explorerUrl}/address/${address}`;
 }
 
 function isoDate(timestamp) {
@@ -194,7 +199,7 @@ function show() {
                     : null;
         const tokenHolders = tb && tb.holders != null ? tb.holders : null;
         const dot = addressDotHtml(tokenId);
-        const tokenLink = `https://etherscan.io/token/${escapeHtml(tokenId)}`;
+        const tokenLink = `${currentNetwork().explorerUrl}/token/${escapeHtml(tokenId)}`;
         const projectUrl = knownToken && knownToken.url ? knownToken.url : null;
         let infoHtml = `<div class="font-bold mb-2">Contract Address</div>`;
         infoHtml +=
@@ -381,7 +386,7 @@ function init(_ctx) {
         let staticHtml = `<div class="font-bold">${escapeHtml(currentSymbol)}</div>`;
         if (tokenId !== "ETH") {
             const dot = addressDotHtml(tokenId);
-            const link = `https://etherscan.io/token/${tokenId}`;
+            const link = `${currentNetwork().explorerUrl}/token/${tokenId}`;
             const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
             staticHtml +=
                 `<div class="flex items-center text-xs">${dot}` +
diff --git a/src/popup/views/approval.js b/src/popup/views/approval.js
index 10de99b..fb95e1b 100644
--- a/src/popup/views/approval.js
+++ b/src/popup/views/approval.js
@@ -7,7 +7,7 @@ const {
     showError,
     hideError,
 } = require("./helpers");
-const { state, saveState } = require("../../shared/state");
+const { state, saveState, currentNetwork } = require("../../shared/state");
 const { formatEther, formatUnits, Interface, toUtf8String } = require("ethers");
 const { ERC20_ABI } = require("../../shared/constants");
 const { TOKEN_BY_ADDRESS } = require("../../shared/tokenList");
@@ -27,7 +27,7 @@ const erc20Iface = new Interface(ERC20_ABI);
 
 function approvalAddressHtml(address) {
     const dot = addressDotHtml(address);
-    const link = `https://etherscan.io/address/${address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${address}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     const title = addressTitle(address, state.wallets);
     let html = "";
@@ -53,7 +53,7 @@ function tokenLabel(address) {
 }
 
 function etherscanTokenLink(address) {
-    return `https://etherscan.io/token/${address}`;
+    return `${currentNetwork().explorerUrl}/token/${address}`;
 }
 
 // Try to decode calldata using known ABIs.
diff --git a/src/popup/views/confirmTx.js b/src/popup/views/confirmTx.js
index 42d984d..be61e71 100644
--- a/src/popup/views/confirmTx.js
+++ b/src/popup/views/confirmTx.js
@@ -20,7 +20,7 @@ const {
     addressDotHtml,
     escapeHtml,
 } = require("./helpers");
-const { state } = require("../../shared/state");
+const { state, currentNetwork } = require("../../shared/state");
 const { getSignerForAddress } = require("../../shared/wallet");
 const { decryptWithPassword } = require("../../shared/vault");
 const { formatUsd, getPrice } = require("../../shared/prices");
@@ -51,11 +51,11 @@ function restore() {
 }
 
 function etherscanTokenLink(address) {
-    return `https://etherscan.io/token/${address}`;
+    return `${currentNetwork().explorerUrl}/token/${address}`;
 }
 
 function etherscanAddressLink(address) {
-    return `https://etherscan.io/address/${address}`;
+    return `${currentNetwork().explorerUrl}/address/${address}`;
 }
 
 function blockieHtml(address) {
diff --git a/src/popup/views/home.js b/src/popup/views/home.js
index d6c2f73..68171d3 100644
--- a/src/popup/views/home.js
+++ b/src/popup/views/home.js
@@ -11,7 +11,12 @@ const {
     escapeHtml,
     truncateMiddle,
 } = require("./helpers");
-const { state, saveState, currentAddress } = require("../../shared/state");
+const {
+    state,
+    saveState,
+    currentAddress,
+    currentNetwork,
+} = require("../../shared/state");
 const {
     updateSendBalance,
     renderSendTokenSelect,
@@ -82,7 +87,7 @@ function renderActiveAddress() {
     if (state.activeAddress) {
         const addr = state.activeAddress;
         const dot = addressDotHtml(addr);
-        const link = `https://etherscan.io/address/${addr}`;
+        const link = `${currentNetwork().explorerUrl}/address/${addr}`;
         el.innerHTML =
             `<span class="underline decoration-dashed cursor-pointer" id="active-addr-copy">${dot}${escapeHtml(addr)}</span>` +
             `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
diff --git a/src/popup/views/receive.js b/src/popup/views/receive.js
index 17103e3..59bac3e 100644
--- a/src/popup/views/receive.js
+++ b/src/popup/views/receive.js
@@ -6,7 +6,7 @@ const {
     formatAddressHtml,
     addressTitle,
 } = require("./helpers");
-const { state, currentAddress } = require("../../shared/state");
+const { state, currentAddress, currentNetwork } = require("../../shared/state");
 const QRCode = require("qrcode");
 
 const EXT_ICON =
@@ -25,7 +25,8 @@ function show() {
         ? formatAddressHtml(address, ensName, null, title)
         : "";
     $("receive-address-block").dataset.full = address;
-    const link = address ? `https://etherscan.io/address/${address}` : "";
+    const net = currentNetwork();
+    const link = address ? `${net.explorerUrl}/address/${address}` : "";
     $("receive-etherscan-link").innerHTML = link
         ? `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`
         : "";
@@ -52,7 +53,9 @@ function show() {
         warningEl.textContent =
             "This is an ERC-20 token. Only send " +
             symbol +
-            " on the Ethereum network to this address. Sending tokens on other networks will result in permanent loss.";
+            " on " +
+            currentNetwork().name +
+            " to this address. Sending tokens on other networks will result in permanent loss.";
         warningEl.style.visibility = "visible";
     } else {
         warningEl.textContent = "";
diff --git a/src/popup/views/send.js b/src/popup/views/send.js
index 6778405..ada1370 100644
--- a/src/popup/views/send.js
+++ b/src/popup/views/send.js
@@ -7,7 +7,7 @@ const {
     addressTitle,
     escapeHtml,
 } = require("./helpers");
-const { state, currentAddress } = require("../../shared/state");
+const { state, currentAddress, currentNetwork } = require("../../shared/state");
 let ctx;
 const { getProvider } = require("../../shared/balances");
 const { KNOWN_SYMBOLS, resolveSymbol } = require("../../shared/tokenList");
@@ -149,7 +149,7 @@ function updateSendBalance() {
     const addr = currentAddress();
     if (!addr) return;
     const dot = addressDotHtml(addr.address);
-    const link = `https://etherscan.io/address/${addr.address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${addr.address}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     const title = addressTitle(addr.address, state.wallets);
     let fromHtml = "";
diff --git a/src/popup/views/settings.js b/src/popup/views/settings.js
index 9263c46..9bae55a 100644
--- a/src/popup/views/settings.js
+++ b/src/popup/views/settings.js
@@ -1,7 +1,7 @@
 const { $, showView, showFlash, escapeHtml } = require("./helpers");
 const { applyTheme } = require("../theme");
-const { state, saveState } = require("../../shared/state");
-const { ETHEREUM_MAINNET_CHAIN_ID } = require("../../shared/constants");
+const { state, saveState, currentNetwork } = require("../../shared/state");
+const { NETWORKS, SUPPORTED_CHAIN_IDS } = require("../../shared/networks");
 const { log, debugFetch } = require("../../shared/log");
 const deleteWallet = require("./deleteWallet");
 
@@ -125,6 +125,10 @@ function renderWalletListSettings() {
 function show() {
     $("settings-rpc").value = state.rpcUrl;
     $("settings-blockscout").value = state.blockscoutUrl;
+    const networkSelect = $("settings-network");
+    if (networkSelect) {
+        networkSelect.value = state.networkId;
+    }
     renderTrackedTokens();
     renderSiteLists();
     renderWalletListSettings();
@@ -168,9 +172,12 @@ function init(ctx) {
                 showFlash("Endpoint returned error: " + json.error.message);
                 return;
             }
-            if (json.result !== ETHEREUM_MAINNET_CHAIN_ID) {
+            const net = currentNetwork();
+            if (json.result !== net.chainId) {
                 showFlash(
-                    "Wrong network (expected mainnet, got chain " +
+                    "Wrong network (expected " +
+                        net.name +
+                        ", got chain " +
                         json.result +
                         ").",
                 );
@@ -209,6 +216,22 @@ function init(ctx) {
         showFlash("Saved.");
     });
 
+    const networkSelect = $("settings-network");
+    if (networkSelect) {
+        networkSelect.addEventListener("change", async () => {
+            const newId = networkSelect.value;
+            const net = NETWORKS[newId];
+            if (!net) return;
+            state.networkId = newId;
+            state.rpcUrl = net.defaultRpcUrl;
+            state.blockscoutUrl = net.defaultBlockscoutUrl;
+            $("settings-rpc").value = state.rpcUrl;
+            $("settings-blockscout").value = state.blockscoutUrl;
+            await saveState();
+            showFlash("Switched to " + net.name + ".");
+        });
+    }
+
     $("settings-show-zero-balances").checked = state.showZeroBalanceTokens;
     $("settings-show-zero-balances").addEventListener("change", async () => {
         state.showZeroBalanceTokens = $("settings-show-zero-balances").checked;
diff --git a/src/popup/views/transactionDetail.js b/src/popup/views/transactionDetail.js
index 7af9d50..306a9ac 100644
--- a/src/popup/views/transactionDetail.js
+++ b/src/popup/views/transactionDetail.js
@@ -12,7 +12,7 @@ const {
     isoDate,
     timeAgo,
 } = require("./helpers");
-const { state } = require("../../shared/state");
+const { state, currentNetwork } = require("../../shared/state");
 const { formatEther, formatUnits } = require("ethers");
 const makeBlockie = require("ethereum-blockies-base64");
 const { log, debugFetch } = require("../../shared/log");
@@ -69,7 +69,7 @@ function etherscanLinkHtml(url) {
 function txAddressHtml(address, ensName, title) {
     const blockie = blockieHtml(address);
     const dot = addressDotHtml(address);
-    const link = `https://etherscan.io/address/${address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${address}`;
     const extLink = etherscanLinkHtml(link);
     let html = `<div class="mb-1">${blockie}</div>`;
     if (title) {
@@ -95,7 +95,7 @@ function txAddressHtml(address, ensName, title) {
 }
 
 function txHashHtml(hash) {
-    const link = `https://etherscan.io/tx/${hash}`;
+    const link = `${currentNetwork().explorerUrl}/tx/${hash}`;
     const extLink = etherscanLinkHtml(link);
     return copyableHtml(hash, "break-all") + extLink;
 }
@@ -172,7 +172,7 @@ function render() {
     if (tokenContractSection && tokenContractEl) {
         if (tx.contractAddress) {
             const dot = addressDotHtml(tx.contractAddress);
-            const link = `https://etherscan.io/token/${tx.contractAddress}`;
+            const link = `${currentNetwork().explorerUrl}/token/${tx.contractAddress}`;
             tokenContractEl.innerHTML =
                 `<div class="flex items-center">${dot}` +
                 copyableHtml(tx.contractAddress, "break-all") +
@@ -234,7 +234,7 @@ function showDetailField(sectionId, contentId, value) {
 function populateOnChainDetails(txData) {
     // Block number
     if (txData.block_number != null) {
-        const blockLink = `https://etherscan.io/block/${txData.block_number}`;
+        const blockLink = `${currentNetwork().explorerUrl}/block/${txData.block_number}`;
         const blockSection = $("tx-detail-block-section");
         const blockEl = $("tx-detail-block");
         if (blockSection && blockEl) {
@@ -361,12 +361,12 @@ async function loadFullTxDetails(txHash, toAddress, isContractCall) {
                     if (tokenSymbol) {
                         detailsHtml += `<div class="font-bold">${escapeHtml(tokenSymbol)}</div>`;
                     }
-                    const etherscanUrl = `https://etherscan.io/token/${d.address}`;
+                    const etherscanUrl = `${currentNetwork().explorerUrl}/token/${d.address}`;
                     detailsHtml += `<div class="flex items-center">${dot}${copyableHtml(d.address, "break-all")}${etherscanLinkHtml(etherscanUrl)}</div>`;
                 } else if (d.address) {
                     // Protocol/contract entry: show name + Etherscan link
                     const dot = addressDotHtml(d.address);
-                    const etherscanUrl = `https://etherscan.io/address/${d.address}`;
+                    const etherscanUrl = `${currentNetwork().explorerUrl}/address/${d.address}`;
                     detailsHtml += `<div class="flex items-center">${dot}${copyableHtml(d.value, "break-all")}${etherscanLinkHtml(etherscanUrl)}</div>`;
                 } else {
                     detailsHtml += `<div class="font-bold">${escapeHtml(d.value)}</div>`;
diff --git a/src/popup/views/txStatus.js b/src/popup/views/txStatus.js
index a406dca..ae155fc 100644
--- a/src/popup/views/txStatus.js
+++ b/src/popup/views/txStatus.js
@@ -10,7 +10,7 @@ const {
     escapeHtml,
 } = require("./helpers");
 const { TOKEN_BY_ADDRESS } = require("../../shared/tokenList");
-const { state, saveState } = require("../../shared/state");
+const { state, saveState, currentNetwork } = require("../../shared/state");
 const { getProvider } = require("../../shared/balances");
 const { log } = require("../../shared/log");
 
@@ -38,7 +38,7 @@ function clearTimers() {
 
 function toAddressHtml(address) {
     const dot = addressDotHtml(address);
-    const link = `https://etherscan.io/address/${address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${address}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     const title = addressTitle(address, state.wallets);
     if (title) {
@@ -52,7 +52,7 @@ function toAddressHtml(address) {
 }
 
 function txHashHtml(hash) {
-    const link = `https://etherscan.io/tx/${hash}`;
+    const link = `${currentNetwork().explorerUrl}/tx/${hash}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     return (
         `<span class="underline decoration-dashed cursor-pointer break-all" data-copy="${escapeHtml(hash)}">${escapeHtml(hash)}</span>` +
@@ -62,7 +62,7 @@ function txHashHtml(hash) {
 
 function blockNumberHtml(blockNumber) {
     const num = String(blockNumber);
-    const link = `https://etherscan.io/block/${num}`;
+    const link = `${currentNetwork().explorerUrl}/block/${num}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     return (
         `<span class="underline decoration-dashed cursor-pointer" data-copy="${escapeHtml(num)}">${escapeHtml(num)}</span>` +
@@ -147,7 +147,7 @@ function tokenLabel(address) {
 }
 
 function etherscanTokenLink(address) {
-    return `https://etherscan.io/token/${address}`;
+    return `${currentNetwork().explorerUrl}/token/${address}`;
 }
 
 function decodedDetailsHtml(decoded) {
diff --git a/src/shared/balances.js b/src/shared/balances.js
index 73f047b..46d5c5e 100644
--- a/src/shared/balances.js
+++ b/src/shared/balances.js
@@ -15,10 +15,15 @@ const { KNOWN_SYMBOLS, TOKEN_BY_ADDRESS } = require("./tokenList");
 
 // Use a static network to skip auto-detection (which can fail and cause
 // "could not coalesce error" on some RPC endpoints like Cloudflare).
-const mainnet = Network.from("mainnet");
-
-function getProvider(rpcUrl) {
-    return new JsonRpcProvider(rpcUrl, mainnet, { staticNetwork: mainnet });
+// Accepts an optional networkName ("mainnet" or "sepolia") for the static
+// network hint so ethers picks the right chain parameters.  When omitted,
+// reads the currently selected network from extension state.
+function getProvider(rpcUrl, networkName) {
+    // Lazy require to avoid circular dependency issues at module scope.
+    const { currentNetwork } = require("./state");
+    const name = networkName || currentNetwork().id;
+    const net = Network.from(name);
+    return new JsonRpcProvider(rpcUrl, net, { staticNetwork: net });
 }
 
 function formatBalance(wei) {
diff --git a/src/shared/constants.js b/src/shared/constants.js
index cf9a661..1280296 100644
--- a/src/shared/constants.js
+++ b/src/shared/constants.js
@@ -3,6 +3,7 @@ const DEBUG_MNEMONIC =
     "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
 
 const ETHEREUM_MAINNET_CHAIN_ID = "0x1";
+const ETHEREUM_SEPOLIA_CHAIN_ID = "0xaa36a7";
 
 const DEFAULT_RPC_URL = "https://ethereum-rpc.publicnode.com";
 
@@ -37,6 +38,7 @@ module.exports = {
     DEBUG,
     DEBUG_MNEMONIC,
     ETHEREUM_MAINNET_CHAIN_ID,
+    ETHEREUM_SEPOLIA_CHAIN_ID,
     DEFAULT_RPC_URL,
     DEFAULT_BLOCKSCOUT_URL,
     BIP44_ETH_PATH,
diff --git a/src/shared/etherscanLabels.js b/src/shared/etherscanLabels.js
index 9c8c658..e74e884 100644
--- a/src/shared/etherscanLabels.js
+++ b/src/shared/etherscanLabels.js
@@ -2,8 +2,6 @@
 // Extension users make the requests directly to Etherscan — no proxy needed.
 // This is a best-effort enrichment: network failures return null silently.
 
-const ETHERSCAN_BASE = "https://etherscan.io/address/";
-
 // Patterns in the page title that indicate a flagged address.
 // Title format: "Fake_Phishing184810 | Address: 0x... | Etherscan"
 const PHISHING_LABEL_PATTERNS = [/^Fake_Phishing/i, /^Phish:/i, /^Exploiter/i];
@@ -74,12 +72,19 @@ function parseEtherscanPage(html) {
  * Returns a warning object if the address is flagged, or null.
  * Network failures return null silently (best-effort check).
  *
+ * Uses the current network's explorer URL so the lookup works on both
+ * mainnet (etherscan.io) and Sepolia (sepolia.etherscan.io).
+ *
  * @param {string} address - Ethereum address to check.
  * @returns {Promise<{type: string, message: string, severity: string}|null>}
  */
 async function checkEtherscanLabel(address) {
     try {
-        const resp = await fetch(ETHERSCAN_BASE + address, {
+        // Lazy require to avoid pulling in chrome.storage at module scope
+        // (which breaks unit tests that only exercise parseEtherscanPage).
+        const { currentNetwork } = require("./state");
+        const etherscanBase = currentNetwork().explorerUrl + "/address/";
+        const resp = await fetch(etherscanBase + address, {
             headers: { Accept: "text/html" },
         });
         if (!resp.ok) return null;
diff --git a/src/shared/networks.js b/src/shared/networks.js
new file mode 100644
index 0000000..4550590
--- /dev/null
+++ b/src/shared/networks.js
@@ -0,0 +1,57 @@
+// Network definitions for supported Ethereum networks.
+// Each network specifies its chain ID, default RPC and Blockscout endpoints,
+// and the block explorer base URL used for address/tx/token/block links.
+
+const NETWORKS = {
+    mainnet: {
+        id: "mainnet",
+        name: "Ethereum Mainnet",
+        chainId: "0x1",
+        networkVersion: "1",
+        nativeCurrency: "ETH",
+        defaultRpcUrl: "https://ethereum-rpc.publicnode.com",
+        defaultBlockscoutUrl: "https://eth.blockscout.com/api/v2",
+        explorerUrl: "https://etherscan.io",
+        isTestnet: false,
+    },
+    sepolia: {
+        id: "sepolia",
+        name: "Sepolia Testnet",
+        chainId: "0xaa36a7",
+        networkVersion: "11155111",
+        nativeCurrency: "SepoliaETH",
+        defaultRpcUrl: "https://ethereum-sepolia-rpc.publicnode.com",
+        defaultBlockscoutUrl: "https://eth-sepolia.blockscout.com/api/v2",
+        explorerUrl: "https://sepolia.etherscan.io",
+        isTestnet: true,
+    },
+};
+
+const SUPPORTED_CHAIN_IDS = new Set(
+    Object.values(NETWORKS).map((n) => n.chainId),
+);
+
+function networkById(id) {
+    return NETWORKS[id] || NETWORKS.mainnet;
+}
+
+function networkByChainId(chainId) {
+    for (const net of Object.values(NETWORKS)) {
+        if (net.chainId === chainId) return net;
+    }
+    return null;
+}
+
+// Build a block explorer link for the given path type and value.
+// type: "address" | "tx" | "token" | "block"
+function explorerLink(network, type, value) {
+    return `${network.explorerUrl}/${type}/${value}`;
+}
+
+module.exports = {
+    NETWORKS,
+    SUPPORTED_CHAIN_IDS,
+    networkById,
+    networkByChainId,
+    explorerLink,
+};
diff --git a/src/shared/prices.js b/src/shared/prices.js
index 61584ff..363f40c 100644
--- a/src/shared/prices.js
+++ b/src/shared/prices.js
@@ -8,6 +8,9 @@ const prices = {};
 let lastFetchedAt = 0;
 
 async function refreshPrices() {
+    // Testnet tokens have no real market value — skip price fetching.
+    const { currentNetwork } = require("./state");
+    if (currentNetwork().isTestnet) return;
     const now = Date.now();
     if (now - lastFetchedAt < PRICE_CACHE_TTL) return;
     try {
diff --git a/src/shared/state.js b/src/shared/state.js
index 307a339..b39faed 100644
--- a/src/shared/state.js
+++ b/src/shared/state.js
@@ -1,6 +1,7 @@
 // State management and extension storage persistence.
 
 const { DEFAULT_RPC_URL, DEFAULT_BLOCKSCOUT_URL } = require("./constants");
+const { networkById } = require("./networks");
 
 const storageApi =
     typeof browser !== "undefined"
@@ -11,6 +12,7 @@ const DEFAULT_STATE = {
     hasWallet: false,
     wallets: [],
     trackedTokens: [],
+    networkId: "mainnet",
     rpcUrl: DEFAULT_RPC_URL,
     blockscoutUrl: DEFAULT_BLOCKSCOUT_URL,
     lastBalanceRefresh: 0,
@@ -38,11 +40,17 @@ const state = {
     viewData: {},
 };
 
+// Return the network configuration for the currently selected network.
+function currentNetwork() {
+    return networkById(state.networkId);
+}
+
 async function saveState() {
     const persisted = {
         hasWallet: state.hasWallet,
         wallets: state.wallets,
         trackedTokens: state.trackedTokens,
+        networkId: state.networkId,
         rpcUrl: state.rpcUrl,
         blockscoutUrl: state.blockscoutUrl,
         lastBalanceRefresh: state.lastBalanceRefresh,
@@ -75,6 +83,7 @@ async function loadState() {
         state.hasWallet = saved.hasWallet;
         state.wallets = saved.wallets || [];
         state.trackedTokens = saved.trackedTokens || [];
+        state.networkId = saved.networkId || DEFAULT_STATE.networkId;
         state.rpcUrl = saved.rpcUrl || DEFAULT_STATE.rpcUrl;
         state.blockscoutUrl =
             saved.blockscoutUrl || DEFAULT_STATE.blockscoutUrl;
@@ -134,4 +143,10 @@ function currentAddress() {
     return state.wallets[state.selectedWallet].addresses[state.selectedAddress];
 }
 
-module.exports = { state, saveState, loadState, currentAddress };
+module.exports = {
+    state,
+    saveState,
+    loadState,
+    currentAddress,
+    currentNetwork,
+};