This paper looks at one of the most important alterations: the move to computer-based assessment in 2015. Between 2000 and 2012, PISA was carried out as a regular paper-based assessment. However, in 2015, pupils in the great majority of countries instead took the test on a computer. Since the change to computer-based assessment could affect pupil performance by itself – in ways that differ between countries – it has the potential to reduce comparability of PISA test scores across countries and over time.