TALIS has been conducted every five years since 2008, with TALIS 2018 being the third cycle. The study has given teachers and school leaders the opportunity to voice their opinions on their working conditions, learning environments and practices. The perspective of teachers and school leaders is vital for education systems to understand what is taking place in their schools and classrooms, and to guide the development of policy. This "Teaching in Focus" brief introduces the research scope and design of TALIS 2018. It also describes the content covered in Volume I of the results and the main research questions that the study attempts to answer.