The purpose of this article is to analyse the assumptions regarding how the Programme for International Student Assessment (PISA) is to achieve its intended effects, that is, to reconstruct PISA’s programme theory (PT) and to probe the validity of its underlying assumptions. The article demonstrates that PISA’s PT has low internal validity. PISA results to react to and reflect on their own practice, compare themselves with others, and then act accordingly to improve education systems and school practice, though no activities or resources are allocated to change school practice.