There are numerous technical and organizational challenges in the design and implementation of spatial decision support systems. Part of the problem stems from the distributed and uncoordinated land management practices of individual decision-makers. For example, in environmental planning, multiple decision makers with conflicting goals may need to make collective decisions. This requires collaborative decision-making tools and conflict resolution capabilities. In this chapter, we identify the research issues related to the design and implementation of Web- based collaborative spatial decision-making support systems in the context of distributed environmental planning. We implemented a Web-based Spatial Decision Support System called GEO-ELCA (Exploratory Land Use Change Assessment) for typical decision-making tasks by urban or municipal planning agencies where resource managers or stakeholders of different interest groups can express their options for future land use changes and assess the resulting hydrological impacts in a collaborative environment.