diff --git a/README.md b/README.md
index a13d38a..7c3629f 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,22 @@ table in real time: it streams BGP UPDATE messages from the RIPE RIS Live
 service, maintains a live routing table in SQLite, and provides HTTP APIs for
 querying routing information.
 
+## Rationale
+
+RIPE RIS Live streams the BGP updates that RIPE's route collectors receive, as
+they happen, from the moment a client connects. It reports changes, not the
+routing table they add up to, so it cannot say which AS an IP address is routed
+to now or which routes a prefix has. RouteWatch applies the feed's updates to
+live routing tables in its own SQLite database and answers IP-to-AS lookups and
+prefix pages from them.
+
+The feed sends no table when a client connects, so these tables hold only the
+routes the feed has announced since RouteWatch first started: an IP-to-AS lookup
+finds nothing for an address whose routes have not been announced since then.
+The tables also miss the updates RouteWatch drops when it falls behind under
+load and those sent while it is stopped, so a lookup or a prefix page can show a
+route the feed has withdrawn.
+
 ## Features
 
 - Real-time streaming of BGP updates from RIPE RIS Live