In a complex, multisite initiative, a well-developed theory of change can help an organization's leadership make strategic decisions about program elements, organizational direction and priorities, as well as provide structure and accountability for evaluation. This article describes the multistep process used by the evaluation team of a large nonprofit to iteratively develop a theory of change for a complex national initiative, including challenges encountered during the creation process and the initial implementation phase. We describe the benefits derived from the development process and the uses of the theory of change by the program staff and by the evaluation team. Although the theory of change highlighted weaknesses in the evaluation, it also provided us with an opportunity to improve the evaluation. Recommendations are included for those evaluators seeking to develop a theory of change for their own organizations.