Grouping pupils by attainment is frequently practised in primary schools yet is associated with detrimental effects for middle- and lower-attaining children. Drawing on a mixed methods study, we find that attainment grouping practices at key stage 2 in primary schools are seldom straightforward. Although grouping by attainment appears to be the dominant form of grouping, the language used by teachers to talk about their classroom practice suggests a varied and sometimes complex picture. We explore how school leaders and teachers justify their grouping practices and conclude that primary school educators endeavour to strike a balance between their concern for the child and the need to respond to the demands of testing and assessment. In the wake of new reforms to primary education, the findings in this study are significant and timely in providing a picture of the types of grouping currently being carried out in primary schools across England.