This publication presents the theory underlying the PISA 2015 assessment – the sixth since the programme’s inception. It includes frameworks for assessing the three core subjects – science, reading and mathematics (Chapters 2, 3 and 4, respectively) – and the framework for an assessment of students’ financial literacy (Chapter 5). This revised edition includes the framework for assessing collaborative problem solving, outlined in Chapter 7. The chapters outline the knowledge content that students need to acquire in each domain, the processes that students need to be able to perform, and the contexts in which this knowledge and these skills are applied. They also discuss how each domain is assessed. Chapter 6 explains the theory underlying the context questionnaires distributed to students, their parents, school principals and teachers.