If a creature is invisible, it has several advantages against creatures that can’t see it: It has total concealment against them, it doesn’t provoke opportunity attacks from them, and they grant combat advantage to it.

Published in Player's Handbook, page(s) 281, Player's Handbook 3, page(s) 221, Rules Compendium, page(s) 313, Monster Vault: Threats to the Nentir Vale, page(s) 124.