This chapter outlines the theoretical framework used in the book. It looks at the concept of “subject” used by Foucault as a way of describing an individual in society. Foucault talked of his academic work as centering around the idea of how subjects are made in interaction with others. The chapter provides examples of research that have used this idea in mathematics education and elsewhere. The chapter then explores the related idea of subjectivity as something that a subject does – explaining the difference between this and the more popularly recognized concept of identity which is seen as fixed – something one has. The chapter then describes Foucault’s concept of subjectification as a process of subjects making themselves accountable to discourses which recognise and make them visible. Finally, the chapter looks at Foucault’s view of discourse – the things we say and the practices such saying allows – as the means by which subjects, subjectivity, and subjectification are made.