Interlocal cooperation through service-sharing agreements has a long history, but its use has increased in popularity during the last 20 years. The decisions of local government units to collaborate through intergovernmental service agreements are best understood as a two-stage process. The first stage, in which communities decide whether to consider interlocal cooperation, involves the nature of the immediate problem faced plus specific demands for performance and efficiency gains that can result from service cooperation. In the second stage, communities confront a question of institutional supply, and hence must overcome inherent bargaining and collective action issues in order to forge interlocal agreements. Heckman probit estimates of such complex relationships using data drawn from a 2003 ICMA survey suggest strong support for this model. The authors conclude by discussing the role of network relationships among local actors for reducing transaction costs and facilitating intergovernmental collaboration.