This issue explores the current state of program theory as it has developed over the last ten years and examines the role program theory can play in a range of new areas that have recently arisen in evaluation. The contributors draw on a review of the literature to discuss the history of program evaluation, its diversity in application, and its strength and limitations in practice, including the special challenges related to causal inference. They describe opportunities for program theory to help evaluators in areas such as measuring performance, replicating successful programs, helping program staff understand the support evaluation, and conducting meta-analysis. They outline a future agenda for program theory evaluation, explain why it is essential that there be an increase in real-world tests and applications, and more. This is the 87th issue of the quarterly journal "New Directions for Evaluation."