This publication from the Organisation for Economic Cooperation and Development presents the theory underlying the Program for International Student Assessment (PISA), including a re-developed and expanded framework for  reading literacy, which incorporates an innovative component on the capacity to read and understand electronic texts, thus reflecting the importance of information and computer technologies in modern societies. It also provides the basis for the assessment of mathematics and science. Within each domain, the knowledge content that students need to acquire is defined, as well as the processes that need to be performed and the contexts in which knowledge and skills are applied. It also illustrates the domains and their aspects with sample tasks. Finally, the theory underlying the context questionnaires is presented. These are used to gather information from students, schools and parents on the students’ home background and attitudes, their learning histories, and their learning environments at school.