Governments and other stakeholders have become increasingly interested in assessing the skills of their adult populations in order to monitor how well prepared they are to meet the challenges of the new information world. The current paper provides an overview of the conceptual framework for the assessment of numeracy developed for the OECD’s Programme for the International Assessment of Adult Competencies (PIAAC). This builds on the numeracy framework developed for the Adult Literacy and Life Skills Survey (ALL). Numeracy is broadly defined and complemented with a definition of ‘numerate behaviour’. Four facets of numerate behaviour are identified and described to guide the development of assessment tasks.