For more than 20 years, my research has focused on the professional knowledge of mathematics teachers. In 2002 this research ‘entered the classroom’, to observe how teachers apply what they know in the act of teaching mathematics. The result was the Knowledge Quartet (KQ), a theoretical framework for analysing and developing mathematics teaching. I shall describe how the KQ emerged, and how the scope of the KQ has since been extended from its origins in English elementary classrooms – including the application of the KQ to the teaching of other disciplines, such as science, technology and English