Being able to address learners' constantly evolving needs requires teachers to keep expanding their knowledge in their respective fields and actively engage in professional development practices. This qualitative meta-synthesis, therefore, aimed to investigate the research studies published between 2006 and 2020 with respect to in-service English language teachers' professional development. In addition to reporting descriptive information of the studies. this synthesis study highlighted directions of research under five main themes: (1) changes in beliefs and practices with a transition from structuralist to communicative orientations; (2) online platforms in professional development; (3) collaborative professional development through research, coaching and study abroad; (4) the intricate relationship between identity and professional development; and (5) the implications for further effective professional development. Finally, this study synthesises fundamental issues in English language teacher professional development and suggests further research ideas in relation to the findings of the present review. Implications for future professional development practices and research are discussed. It is argued that successful professional development recognising teachers as key agents in the process should provide room for contextualising theory and combining it with practice and application in class. Effective professional development also necessitates acknowledging teachers' personal practical knowledge. Professional development building on varied and differentiated activities and taking the particulars of diverse teaching contexts into consideration. would contribute to teacher growth provided particular conditions with an overt emphasis on practice, teacher needs, collaboration and sustainability. Such synthesis studies would broaden our understanding of English language teacher professional development and prompt necessary actions for development in educational systems.