The purpose of this paper is to consider the role of mathematics teachers’ thinking and decision making in the classroom. This has been a somewhat neglected area of research since the mid-1980s, but I will argue here that understanding the nature of teachers’ thinking and decision making in lessons is important in understanding practice and can inform approaches to initial teacher education and professional development. While mathematics teachers’ knowledge and beliefs are important, the decisions they make and the actions they implement in the lesson influence the learning environment, culture and interpretation of tasks and activities. I draw on my own empirical research along with a multidisciplinary account based on developments in cognitive psychology, neuroscience and ontology (e.g. posthumanism) to provide a theoretical account of teacher thinking and decision making and how this influences practice.