This book presents the theory behind the development of the 2009 PISA survey. The re-worked and expanded framework for reading literacy includes not only the assessment of reading and understanding printed texts, but also an innovative component to assess how well students read, navigate and understand electronic texts. Additionally, this publication provides the basis for measuring mathematical and scientific competencies. Finally, it presents the theory behind the questionnaires used to gather information from students, schools and parents on students’ home backgrounds, students’ approaches to learning and school learning environments.