Teachers are seen as the cornerstone of quality in education. The quality of education can be improved by increasing the quality of teachers through professional development. In the research, it was aimed to comparatively examine the in-service professional development practices of teachers in Estonia, Sweden, Netherlands, Canada, Japan and Turkey, which participated in the PISA 2018 and TALIS 2018 international surveys among OECD countries and showed different levels of success in PISA. For this purpose, with Bereday’s analytical method, firstly, the general information of the countries and the in-service professional development practices were examined. After the examination, the data obtained were described. Then the in-service professional development practices were interpreted and brought side by side with the tables and finally, the similarities and differences between them were interpreted. Although it is seen that education policies are determined centrally by the ministries of education, it is seen that Estonia, Japan and Turkey organize professional development activities at the central level in terms of teachers’ professional development practices, while Sweden, the Netherlands and Canada organize professional development activities at the local level. It has been observed that universities, schools, training centers and digital platforms are used in in-service professional development. It has been seen that in-service training includes up-to-date knowledge and skills in addition to pedagogical, subject area and general culture topics. In addition, it has been observed that in-service training in the form of courses, seminars, conferences, workshops, school visits, literature reviews and learning networks are applied as a type of training.