This chapter presents an approach for the integration of Multi-criteria Evaluation (MCE) into Spatial Data Infrastructures (SDIs) to be used in an ad hoc manner. The approach is demonstrated using two case studies: (1) a selection of action priorities in emergency management, illustrated with a specific implementation for the SoKNOS project; (2) a safe site selection problem that identifies future tasks and issues related to interoperable spatially enabled MCE Services. Special concern is given to interoperability by relating our approach to the current OpenGIS Web Processing Service specification and the ORCHESTRA and INSPIRE architecture frameworks. Finally, this chapter discusses the initial steps required for an application profile for MCE in SDI.