Citizen participation is commonly viewed as attempts to influence the formulation of public policy. In this paper, the author argues that citizens also can and do exert important influences on policy through their participation in the execution of public programs. This is particularly the case in human services where change in the client's behavior is the "product" which is supposed to be delivered. Citizens "coproduce" public services by requesting assistance from service agents, by cooperating with service agents in carrying out agency programs, and by negotiating with service agents to redirect agents' activities. Citizen participation in service delivery is, in fact, often critical to program success.