The Room of Requirement, also known as the Come and Go Room, is a room at Hogwarts that a person can only enter when they have real need of it. When it appears, it is always equipped for the seeker's needs. The room is located on the seventh floor of the castle, opposite the tapestry of Barnabas the Barmy.
Harry hides his copy of Advanced Potion Making in the Room of Requirement after his duel with Draco Malfoy. At Harry's request, the Room of Requirement provides a place for Harry to hide the book: a room the size of a large cathedral with very high windows, and towering walls built of objects hidden by generations of Hogwarts students. The room is filled with alleyways and roads bordered by teetering piles of broken furniture, books, banned objects and other forbidden items.