diff --git a/book/src/user/lookup-tables.md b/book/src/user/lookup-tables.md
index 25e60e9..2709a94 100644
--- a/book/src/user/lookup-tables.md
+++ b/book/src/user/lookup-tables.md
@@ -4,4 +4,8 @@ In normal programs, you can trade memory for CPU to improve performance, by pre-
 and storing lookup tables for some part of the computation. We can do the same thing in
 halo2 circuits!
 
+A lookup table can be thought of as enforcing a *relation* between variables, where the relation is expressed as a table.
+Assuming we have only one lookup argument in our constraint system, the total size of tables is constrained by the size of the circuit:
+each table entry costs one row, and it also costs one row to do each lookup.
+
 TODO