Evaluating complex interventions requires an understanding of the program's logic of action. Logic analysis, a specific type of program theory evaluation based on scientific knowledge, can help identify either the critical conditions for achieving desired outcomes or alternative interventions for that purpose. In this article, we outline the principles of logic analysis and its roots. We then illustrate its use with an actual evaluation case. Finally, we discuss the advantages of conducting logic analysis prior to other types of evaluations. This article will provide evaluators with both theoretical and practical information to help them in conceptualizing their evaluations.