Teachers need support to actively participate in this transformation and demand high standards from all students, while taking into account the increasing diversity in their classrooms. In most countries, teachers undergo pre-service training, followed by a selection process, with subsequent in-service training provided mostly through courses and workshops. New evidence shows that the proportion of certified teachers and most forms of professional development have a weak impact on student performance. Thus, many reforms aim to improve the quality of teacher training, make selection procedures more demanding, develop new forms of professional development and raise curricular standards.