This paper summarizes research on Web-based spatial decision support systems (WebSDSS). The review distinguishes early server-side from more recent client-side applications. A third category of WebSDSS focusing on spatial decision support in public participation is typically implemented as a mixed client/server-based system. Conclusions drawn from previous work include the need for systematic user studies of WebSDSS, and the adoption of interoperable architectures for distributed spatial decision support. Furthermore, a conceptual framework is proposed to facilitate further studies of WebSDSS methods.