Former literature reviews suggested that if (student) teachers learn together in their professional development activities, professional development is enhanced. In the present literature review, we explored a variety of peer teacher professional development activities, conceptually divided into coaching, collaborating, and assessing activities. Specifically, we examined which learning outcomes could be achieved through participating in such activities. By means of a meta-study approach, we additionally reviewed methods, data collection, and theories used in the included studies. Findings showed outcomes were achieved in terms of teacher knowledge, teacher skills, and student learning. At the same time, many studies had methodological weaknesses: many self-report and self-constructed instruments hardly based on theory were applied. Based on the findings, we urge for more rigorous studies that can move the field forward.