Teachers’ own learning is an integral part of their practice – after all, it is said that to teach is to learn twice over. But understanding which kinds of in-service training are effective for teachers and which aren’t can be complex, and many countries have yet to find the perfect formula. Data can provide some guidance. Edmund Mission, Deputy CEO of the Australian Institute for Teaching and School Leadership (AITSL), and Pablo Fraser, an analyst for the OECD’s Teaching and Learning International Survey (TALIS), discuss the importance of professional development for teachers and the direction systems should be heading in to get the most out of their professional development programmes.