High-performing countries use various mechanisms to select the best candidates to the teaching profession. In Finland, Hong-Kong (China), Macao (China) and Chinese Taipei, students who wish to enter teacher-training programmes must pass a competitive entry examination. In Japan, teaching graduates must pass a competitive examination to start teaching and in Singapore, they must complete a probation period. These requirements, however, are also found among some low-performing countries suggesting that early selection, while important, is not enough to ensure a highly qualified teaching force. Across OECD countries, the proportion of fully certified teachers has a positive, albeit modest association with student performance in PISA. In countries that performed above the OECD average in science, at least 80% of the students are in schools that invite specialists to conduct teacher training or organise in-service workshops for teachers or where teachers cooperate with each other. This is higher, on average, than what is observed among other countries.