A need for evaluating maritime spatial planning (MSP) has become more important as countries are preparing and implementing their MSP. Evaluations are opportunities to foster learning and to accumulate knowledge in order to improve the processes and effectiveness of MSP. Empirical evidence has shown that MSP processes are run in different ways and for different purposes. To address this diversity, this chapter presents a methodology for designing a flexible and context-specific evaluation of MSP. This chapter introduces a theory-based evaluation approach that aims to explain how and why the evaluated planning process or plan works or fails to work. It scrutinises how MSP produces intended and unintended effects, for whom and in what contexts and what mechanisms are triggered by the MSP.