Professional development programs are based on different theories of how students learn and different theories of how teachers learn. Reviewers often sort programs according to design features such as program duration, intensity, or the use of specific techniques such as coaches or online lessons, but these categories do not illuminate the programs' underlying purpose or premises about teaching and teacher learning. This review sorts programs according to their underlying theories of action, which include (a) a main idea that teachers should learn and (b) a strategy for helping teachers enact that idea within their own ongoing systems of practice. Using rigorous research design standards, the review identifies 28 studies. Because studies differ in multiple ways, the review presents program effects graphically rather than statistically. Visual patterns suggest that many popular design features are not associated with program effectiveness. Furthermore, different main ideas are not differentially effective. However, the pedagogies used to facilitate enactment differ in their effectiveness. Finally, the review addresses the question of research design for studies of professional development and suggests that some widely favored research designs might adversely affect study outcomes.