This article presents findings from a comparative study of sources of educational disadvantage of immigrant children across 18 OECD countries, which is based the data from the 2006 Programme for International Student Assessment (PISA). The findings show that disadvantaged family background and lack of host-country-specific cultural capital account for a large part of the attainment gap between immigrants and their non-migrant peers. The findings also show that school characteristics in terms of their size, quality of teachers and educational resources contribute to the understanding of the further part of the immigrant performance gap. Moreover, school characteristics mediate between the immigrant students' family characteristics and their attainment, by reinforcing or diminishing the impact of the family characteristics. Furthermore, the institutional characteristics of immigration countries, such as type of education provision, type of welfare provision and type of immigration policy, also play a part in producing and maintaining educational disadvantage of immigrant pupils, by affecting the attainment level and mediating between the individual- and school-level characteristics and pupils' attainment. It was found that the first generation of immigrant children perform particularly well in countries with a liberal type of welfare regime, more standardised educational systems and more selective immigration policies; there was also some evidence that institutional factors shape educational attainment of the second generation of immigrant children in a way which more closely resembles that of the children from non-immigrant backgrounds--the former perform better in countries with a more inclusive (social-democratic) type of welfare provision, but also in countries with less differentiated and more standardised educational systems. (Contains 7 figures, 4 tables, and 4 notes.)