This collection of essays underscores the importance of theory in program evaluation and analyzes the nature and implications of theory-driven evaluations. Scholars from different disciplines cover a wide range of issues, such as functions of program theory, the interface between theory and methods, strategies for formulating theory, cost and resource constraints, different types of theory-driven evaluations, and the future implications of such evaluations. This broad-based analysis is designed for inter-disciplinary audiences in policy studies and all the social sciences.