The Room of Requirement is a secret room in Hogwarts that only appears when a person is in great need of it. The room then transforms itself into whatever the witch or wizard needs it to be at that moment. It is believed that the room is Unplottable, as it does not appear on the Marauder's Map, nor do its occupants, although this could simply be because James Potter, Remus Lupin, Sirius Black, and Peter Pettigrew never found the room.
