In the previous two chapters we examined a wide range of research reporting inequalities in schools, both at the level of school organization and at that of classroom interaction. Our primary focus there was on how effectively these studies had established that unequal treatment of students from different social categories had occurred. However, most of this research has been concerned not simply with the fact of differential treatment, but also with its effects on educational outcomes. Indeed, as we saw in Chapter 1, this work emerged as part of a movement which rejected earlier explanations for inequalities of outcome, in terms of differences in home background, in favour of an emphasis on the role of school processes. In this chapter, then, we will consider the explanatory validity of research on inequalities in schools; in other words, we will assess how effectively it explains differences in the educational outcomes achieved by different categories of student.