This chapter describes the purpose of the Framework for the PISA surveys of mathematical literacy and its evolution from 2000 to 2012. It also describes some of the analysis and scholarship on which the key constructs of the Framework are based, and links to kindred concepts in the wider mathematics education literature. The chapter does not intend to present the Framework but instead to share insights into its creation by successive Mathematics Expert Groups. The main Framework concept is that of mathematical literacy which has its roots in recognition of the increasing importance of mathematical proficiency in the modern world. The chapter describes mathematical literacy, its evolving definition and the origin of the term within broadened notions of literacies and its relationship to other terms such as quantitative literacy and numeracy. It describes the central constructs of the Framework, which are used to describe what abilities make up mathematical literacy and are also used to ensure that the item pool is comprehensive and balanced. These are the real-world context categories that group the source of mathematical challenges, the phenomenologically-based content categories, the fundamental mathematical capabilities and a set of processes based on the mathematical modelling cycle. The way in which new technologies have expanded the view of mathematical literacy and how this has been assessed through the 2012 computer-based assessment of mathematics is also discussed.