Community partnerships are an increasingly popular strategy for improving community health. This popularity is based less on evidence than on rhetoric. This research developed and tested a systems model of partnership to improve the practice of collaboration in public health. Basing the need for partnerships on the multi-sectoral nature of health, the model used open systems theory to set out requirements for partnership. Institutional theory suggested that problems faced by partnerships may result from partners meeting requirements for legitimacy. Change is, therefore, required, both in organizations and in their institutional environment. Using exploratory case studies, the study design involved site visits to two community health partnerships (West Virginia Community Voices and Healthy New Orleans). Mixed qualitative methods included semi-structured interviews, focus groups, and document review. Analysis involved interpreting informants responses in terms of evidence representing the model and for new elements. Evidence from practice suggested several revisions to the model. One involved applying a typology of organizational affiliation, with partnership toward one end of the continuum. Use of this typology permitted an extension of the model to understand the form of affiliation practiced by Community Voices and of Healthy New Orleans. Multiple opportunities to network and build coalitions in Community Voices led to increased chances of success in achieving health improvement goals. Networking opportunities for individual volunteers led to an informal Healthy New Orleans organization. Results of this research led to an analytic fit between the two sites and the community health partnership model. Recommendations are offered for practice, research, and for funding agencies. With further research, the model can be used to develop practical tools to guide and assess partnerships as a strategy to improve health, as well as to identify environmental barriers to partnership and strategies for change.