This article reviews the history of the measure of socio-economic status in PISA and identifies theoretical underpinnings of the index of economic, social and cultural status (ESCS). It then highlights multiple changes in the instruments and scaling methods used by PISA over time, and suggests ways of resolving the tensions behind some of these changes and thereby stabilise the measure of ESCS. A stable definition and operational procedure to derive the ESCS index appears essential to compare the ESCS-achievement relationship over time. Some of the suggestions included in this article were already implemented in the 2018 cycle.