The multifaceted challenges of contemporary governance demand a complex account of the ways in which those who are subject to laws and policies should participate in making them. This article develops a framework for understanding the range of institutional possibilities for public participation. Mechanisms of participation vary along three important dimensions: who participates, how participants communicate with one another and make decisions together, and how discussions are linked with policy or public action. These three dimensions constitute a space in which any particular mechanism of participation can be located. Different regions of this institutional design space are more and less suited to addressing important problems of democratic governance such as legitimacy, justice, and effective administration.