Postgres Concatenation

I'm trying to a simple concatenation in PostgreSQL and it keeps up throwing up an error message. I don't understand what I am doing wrong here.

select concat('abcde', 'fgh');
No function matches the given name and argument types. You might need to add explicit type casts.

select concat(cast('abcde' as text), cast('fgh' as text));
No function matches the given name and argument types. You might need to add explicit type casts.

I am using Postgres version 8.4.11. Please let me know what is going on.