The need for greater public involvement in environmental decision-making has been highlighted in recent high-profile research reports and emphasized by leaders at all levels of government. In some cases, agencies have opened the door to greater participation in their programs. However, there is relatively little information on what can be gained from greater public involvement and what makes some programs work while others fail. This paper addresses these questions through an evaluation of public participation in environmental planning efforts in the Great Lakes region. The success of participation is measured using five criteria: educating participants, improving the substantive quality of decisions, incorporating public values into decision-making, reducing conflict, and building trust. The paper then discuses the relationship between success and a number of contextual and procedural attributes of a variety of cases. Data come from a "case survey," in which the authors systematically extract information from previously published studies of 30 individual participation cases. The authors conclude that public participation can accomplish important societal goals and that success depends, in large part, on the actions and commitment of government agencies.