In economics, maritime/marine spatial planning (MSP) can be classified as a public choice mechanism designed to alleviate market failure. As such, MSP should ensure rational use or conservation of sea space in situations where the market cannot deliver optimal solutions. In this chapter, both market and public choice mechanisms for allocating sea space are discussed. The final conclusion is that both are closely interrelated. MSP cannot neglect market forces; however, the market itself is unable to allocate space to issues considered important from a societal point of view, such as nature conservation, defence or non-use of the sea.