This study presents findings on the effectiveness and perception of teacher professional development in online-only and blended learning (BL) formats. The analysis of a total of 115 studies from the research field of online teacher professional development (OTPD) showed that digitized training predominantly led to positive effects on teachers' competencies and teaching practice. Among some of the other positive impacts of OTPD were improvements in students' learning performance, attitudes and motivations, and teachers' self-efficacy; though these aspects were considered much less frequently overall. Despite individual criticisms and suggestions for improvement, OTPD courses were endorsed by teachers. In terms of the take-up amongst participants, online-only and BL formats scored similarly, with the inclusion of synchronous phases, collaborative activities, and support activities proving to be decisive criteria for evaluation. In context of support activities, the involvement of an instructor was found to be effective, although it became clear that it is not only the presence of an instructor that matters, but also the way in which he is involved in the OTPD activity (Lantz-Andersson et al. Teach Teach Educ 75:302-315, 2018 ). A basic open attitude towards OTPD was also held by the training staff, even when complications and difficulties were pointed out.