What knowledge do teachers need for 21st century teaching? Today, teachers have an important role in guiding and shaping students' use of digital tools and optimising the educational benefits of their digital experiences. They are also agents of inclusive, equitable education and ambassadors of embracing diversity as an enriching element of our societies. To fulfil these roles teachers need to be experts of teaching and learning, and base their practice on a specialised and updated body of knowledge. However, there is a great need for a better understanding of the knowledge and skills that teaching in the 21st century requires. This is the ambition for the next cycle of the OECD Teaching and Learning International Survey (TALIS) and its new Teacher Knowledge Survey (TKS) assessment module. Studying teaching as a knowledge profession across education systems is as challenging as it is important. This publication aims to contribute to this challenging endeavour by summarising the state-of-art on teacher knowledge and its measurement across systems. It discusses cutting-edge methodologies and designs and outlines implications for research as well as policies and practices for strengthening knowledge-based and evidence-informed practices in schools.