In the 21st century, teachers' learning is viewed through the lens of sustainable development as a holistic, transformative and collaborative learning process. Acknowledging that teacher professional development is a prerequisite for educational quality, it becomes necessary to look for professional development factors that could be relevant to sustainable professional development. The article explores factors of sustainable professional development of teachers based on data of TALIS 2018 from four Baltic countries (Estonia, Finland, Latvia, and Lithuania). The study also discusses distinct characteristics of teacher professional development in the analyzed countries. Although traditional forms and methods of professional development still prevail in all countries, teachers also learn through active cooperation. Finnish teachers, more often than teachers in the other countries, worked in teams and shared material, knowledge, etc. with each other; Estonian teachers, more so than teachers in the other countries, took part in long-term training; Latvian teachers were more likely than teachers in the other countries to observe other teachers' classes and provide feedback. Lithuanian teachers were perhaps the most active in terms of learning, but young teachers with fewer years of service were more likely to engage in long-term and collaborative activities. Finally, the implications for further research are discussed.