The use of ability-grouping is currently increasing in primary schools. Teachers and teacher educators are placed in the unenviable position of having to marry research evidence suggesting that ability-grouping is ineffectual with current policy advocating this approach.This book links theory, policy and practice in a critical examination of ability-grouping practices and their implications in primary schools, with particular reference to primary mathematics. It provides an accessible text for teacher educators to support their students in engaging with the key debates and reflecting upon their practice. Key changes in structural approaches, such as the movement between streaming, setting or mixed-ability teaching arrangements, are explored in the light of political trends, bringing this up to date with a discussion of current policy and practice.