This study explores forces that push public agencies to overcome barriers to cooperation. Case studies of nine groups of local school districts examined the pressures that shaped local participation in cooperative programs. The literature on relationships among organizations suggests six possible reasons to bear the costs of cooperation: to get more resources, to satisfy norms and values, to obtain political advantage, to solve problems, to reduce uncertainty, and to obey legal mandates. Data from the cases illustrate strengths and weaknesses of these six theoretical lines of argument as they apply to the decisions of public managers coping with complex realities. A process model of cooperation integrates the lessons of the data into a perspective that highlights the role of demands for improved performance. The model offers realistic guidance for the design of workable cooperative relationships.