Teachers' teaching-related competence beliefs such as perceived teaching ability and self-efficacy have been linked to their occupational well-being and external evaluations of instructional quality. However, researchers have struggled to establish a reliable empirical link between teachers' competence beliefs and students' academic outcomes. To clarify these puzzling results, this research synthesis reviews different conceptualizations of teachers' competence beliefs and their hypothesized effects on students, and focuses in particular on "student-reported" classroom processes and outcomes in "authentic" K-12 classrooms. This review revealed considerable ambiguity concerning the conceptualization and assessment of teachers' competence beliefs in empirical research. Furthermore, there is a paucity of empirical evidence testing central assumptions about the associations between different types of beliefs about teaching competence, mediating processes such as instructional quality, and student outcomes in authentic K-12 settings. This research synthesis identifies important gaps in existing research that warrant attention and outlines directions for future research.