In this chapter we shall be concerned to articulate and comment on what we see as the central arguments used by teachers and others in education in defence of mixed ability grouping. We recognise that this account must rest in part on empirical studies of what teachers actually say about their reasons, and to this end we shall be drawing on published surveys and case studies. But we shall at the same time try to give fuller articulation to arguments which receive somewhat fragmentary expression in the literature and, in conclusion, to comment on some of their implications and internal contradictions. We shall set out in the following four sections four arguments or sets of arguments which seem to us to have some rather distinctive features.