Multiple meta-analyses have now documented small positive effects of teacher professional development (PD) on pupil test scores. However, the field lacks any validated explanatory account of what differentiates more from less effective in-service training. As a result, researchers have little in the way of advice for those tasked with designing or commissioning better PD. We set out to remedy this by developing a new theory of effective PD based on combinations of causally active components targeted at developing teachers' insights, motivating change, developing teaching techniques, and then embedding these changes in teachers' practice. We test two important implications of the theory using data identified through a systematic review and meta-analysis of 104 randomized controlled trials, finding qualified empirical support for the theory. The main contribution of the article is to provide a testable theory of what makes PD more effective, which can be used to guide future empirical research on this topic.