This chapter explores the development of geographic information systems (GIS)/multi-criteria evaluation (MCE) systems on the web and evaluates their utility as mechanisms for improving public participation in spatial decision making. The rise of the Internet and the World Wide Web has created many opportunities for those involved in GIS research. The main experimental objective of the open spatial decision-making system is to investigate the potential of web-based GIS/MCE decision support tools for improving public understanding of and participation in important environmental decisions. In designing web-based GIS/MCE systems for participatory decision support it is possible to draw on a long and varied history of research into decision making and decision support systems. Use of intelligent spatial agents may be important in ensuring new users and those unfamiliar with spatial science and GIS do not use inappropriate datasets and/or analyses.