Many types of challenging problems faced by decision makers have a geographic or spatial component. Spatial decision support systems (SDSS) can effectively support this class of problem. This represents a growing class of DMSS, taking advantage of the increasing capability of technology to deal with spatial data. SDSS is characterized by the use of significant amounts of public data external to the organizations that use it and the increasing availability of such spatial data facilities wider use of such systems. This chapter describes spatial systems, their history, their relationship to other systems, their mean areas of application and their future development.