Abstract
This chapter focuses on the use of scenarios to inform the development of marine spatial plans. An in-depth example of scenario-building for the Celtic Seas is presented, based on a four-way possibility space using the axes of spatial efficiency and cooperation between planning authorities to create four scenarios which were tested in a workshop. This demonstrated the importance of using scenarios to understand the aspirations of different stakeholders towards integration within the marine spatial planning (MSP) process, the realities of encouraging co-location between sea uses and highlighted important issues where further transboundary cooperation on MSP may be required.