The Room of Requirement, also known as the Come and Go Room, is a hidden chamber at Hogwarts that becomes whatever a person needs it to be. The room is always equipped for the seeker's needs, and only appears (at least to deliberate seekers) when walking past it three times, concentrating on the needs of the seeker. When apparent, the door to the Room of Requiremnet sits in a stretch of otherwise blank wall opposite a tapestry depicting Barnabas the Barmy's attempt to train trolls for the ballet.
 Appearances in the Books
Harry is introduced to the Room of Requirement by Dobby.