"Lesson study" has garnered considerable attention from educational researchers and practitioners as a promising method for improving instruction. At its core, lesson study reflects a collaborative inquiry process, grounded in cycles of planning, enacting, and reflecting, that promotes teacher learning through active engagement in lesson refinement. In this article, we leverage lesson study as an exemplar case to foreground an examination and application of theory-based evaluation. We overview a novel evaluation framework contextualized to this multidimensional instructional program and draw out the contributions of theory-based evaluation to the study of program effects. To ground the description of this framework, we summarize representative findings from our ongoing evaluation of a lesson study-based instructional program and discuss processes for selecting, using, and managing evidence sources as well as determining causal strands supporting program outcomes. We conclude by discussing lessons learned and implications for theory-based evaluation of complex programs.