A bookcase, or bookshelf, is a piece of furniture, almost always with horizontal shelves, used to store books. A bookcase consists of a unit including two or more shelves which may not all be used to contain books or other printed materials. Shelves may be fixed or adjustable to different positions in the case. In rooms entirely devoted to the storage of books they may be permanently fixed to the walls and/or floor. Bookcases frequently have doors that should be closed to protect the books from air pollution, and bookshelves are open-fronted. These doors are almost always glazed, so as to allow the spines of the books to be read. Especially valuable books may be kept in locked cases with wooden or glazed doors. A bookshelf normally stands on some other piece of furniture such as a desk or chest. Larger books are more likely to be kept in horizontal piles and very large books flat on wide shelves.

There are three stationary systems of arranging bookcases: Flat against the wall; in stacks or ranges parallel to each other with merely enough space between to allow of the passage of a librarian; or in bays or alcoves where cases jut out into the room at right angles to the wall-cases. The stack system is suitable only for public libraries where economy of space is essential; the bay system is not only handsome but utilizes the space to great advantage. The library of the City of London at the Guildhall is a peculiarly effective example of the bay arrangement.

For libraries where space is extremely tight there is yet another system, usually called mobile aisle shelving. In such systems rows of bookcases are mounted on wheels and packed tightly together with only one or more aisles between them. It is possible then to visit only two bookcase sides at a time, all the others being pressed close together. A gearing mechanism allows users move the bookcases and open the aisle in the desired location. Because of the danger of tripping on the floor mounted rails or being squashed between bookcases these systems may have electronic sensors and/or recessed track, or are reserved for closed stacks where access is restricted.