Verificate values in other table?

I'm working on a little database and got stuck with the following problem. I
have two tables that look like this

table1 ( ids char(4) )
table2 ( id char(4), comment text)

What I want to do is that if someone adds or updates a row in in table2 I
want to verify that the <id> given exists in table1. And if it doesn't don't
accept the modification and give the user a error message. Is there some way
to do this?

/ Fredrik