Teachers' efficacy beliefs are thought to influence not only their motivation and performance but also the achievement of their students. Scholars have therefore turned their attention toward the sources underlying these important teacher beliefs. This review seeks to evaluate the ways in which researchers have measured and conceptualized the sources of teaching self-efficacy across 82 empirical studies. Specifically, it aims to identify what can be inferred from these studies and what important questions still remain about the origins of teachers' efficacy beliefs. Results indicate that a number of methodological shortcomings in the literature have prevented a clear understanding of how teachers develop a sense of efficacy. Nonetheless, insights gleaned from existing research help to refine, and to expand, theoretical understandings of the sources of self-efficacy and their influence in the unique context of teaching. Implications for future research and practice are addressed.