This paper describes the use of a theory of change approach to community-based cross-agency service planning for children with serious emotional disturbance and their families. Public agency planners in Contra Costa County, California used the theory of change approach to organize service planning for a population of youth who had been arrested and involved with juvenile probation. The theory of change process described in this paper links community outcomes with planned activities with the assumptions or principles that underlie the community planning efforts. When complete, a theory of change logic model can serve as a guide for implementation, ensuring that community plans for service delivery remain true to their intent. The theory of change development process includes twelve stages and is based on a step-by-step approach. Theory of change logic models establish a context for articulating a community's shared beliefs and prompt local stakeholders to establish logical connections between the population to be served, expected results, and strategies intended to achieve those results.