Differentiated instruction is a key indicator of effective teaching, but difficult to implement. Thus, educators have called for professional development (PD) programs on the topic. Based on 27 selected studies, this meta-analysis examined the overall effectiveness of PD programs in terms of teacher and student outcomes, and assessed theoretically relevant PD program features as moderators. Our findings indicated that PD programs have a medium effect on teachers’ knowledge, attitudes and practices. Furthermore, teachers improved more in their differentiation practices if a PD program was provided within a specific subject domain. As PD programs had no significant effect on students’ learning, the relation between teacher development and student learning requires further scrutiny so that students will benefit from their teachers’ learning