The authors provide an overview of the Cooperative Extension System and its program evaluation challenges. Part of the historic land-grant system, Extension exists in all states and territories of the United States and is funded through federal, state, and local (usually county) appropriations, as well as competitive grants and other sources. Complex funding, staffing, and accountability structures combined with widely varying programs and delivery methods make program development and evaluation challenging for Extension. Although each state’s Extension service operates autonomously, they all share a need to communicate program impacts and public value, which has become the main driver for program evaluation. Further, organizational factors such as variation in program evaluation support across states, widely varied evaluation cultures, and the grassroots nature of including stakeholders in program development add even more complexity to evaluating Extension programs.