One of the consistent findings of educational research studies is the effect of the students’ family socio-economic background on their learning achievement. Consequently, international comparative studies emphasise the role of socioeconomic background for determining learning outcomes. In particular, PISA results have been used to describe how different structures of the educational system can mediate the impact of socio-economic family background on performance with comprehensive systems generally providing more equity in educational opportunities. Given the cyclical nature of the PISA study, it is of interest to observe changes in the relationship between socio-economic background and student performance. However, in order to measure the change in a relationship between two variables over time, one needs to rely on the same (or at least very similar) measures for both constructs. Furthermore, there are some concerns regarding the validity of student reports on family background. This paper addresses the issue of measuring socio-economic background in the context of the OECD PISA study. It describes the computation of a composite index of “Economic, Social and Cultural Status” derived from occupational status of parents, educational level of parents and home possessions for the first two PISA cycles. It also shows differences in the relationship between socio-economic background and student performance, both using single-level and multi-level analyses and compares student and parent reports on occupation and education in order to explore the validity of these measures.