Program theory evaluation, which has grown in use over the past 10 years, assesses whether a program is designed in such a way that it can achieve its intended outcomes. This article describes a particular type of program theory evaluation--logic analysis--that allows us to test the plausibility of a program's theory using scientific knowledge. Logic analysis is useful for improving the intervention or finding alternatives for achieving intended outcomes; it influences the choice of type of evaluation to conduct and strengthens the validity of subsequent evaluations. The objective of this article is to present the methodological principles and the roots of this type of program theory evaluation. We illustrate two types of logic analysis with two actual evaluation cases. There are very few published examples of program theory evaluation. This article will provide evaluators with both theoretical and practical information to help them in conceptualizing their evaluations. (Contains 1 figure and 1 table.)