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 emphasis the role of socio-economic 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 system generally providing more equity in educational opportunities. 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 results from the first two PISA surveys regarding the relationship between socio-economic background and student performance, both using single-level and multi-level analysis. Furthermore, it reviews the consistency of student and parent reports on socio-economic based on field trial data from the third PISA cycle collected in 2005. The analysis presented in this paper show that within PISA countries the effects of socio-economic background measures on student performance across the first two PISA cycles remain mostly unchanged. But there is some variation which might partly be due to problem with measurement and researchers as well as policy-makers are strongly advised not to interpret minor changes in the relationship between SES and performance (for example as possible outcomes of recent policy changes). It needs to be recognized that there is a certain amount of measurement error associated with student reports on family background which needs to be taken into account when interpreting findings from PISA. (Contains 13 tables.)