Teacher professional development is deemed to be high quality when it includes opportunities for active learning methods, an extended time period, a group of colleagues, and collective learning activities or research with other teachers. The higher the exposure of teachers to high-quality professional development, the more likely they are to use a wide variety of teaching practices in the classroom. Professional development activities that focus on curriculum knowledge (rather than subject knowledge or pedagogy) and that involve collaborating with other teachers seem particularly well suited to enhancing teachers’ classroom practices. However, these types of professional development are not those that are most widely used around the world. Not all teachers have equal access to high-quality professional development. In some countries and economies, different participation rates in high-quality professional development are observed between male and female teachers, as well as between teachers who have and have not completed initial teacher education.