In asking what we mean when we talk about 'mixed ability groups' or 'mixed ability teaching' we are not announcing an attempt at close conceptual analysis. The concepts are in any case not sufficiently stable or firmly established to lend themselves readily to such analysis. What we do want to begin to identify is something of the range of practice whose logic and rationale it is the purpose of this book to examine. In this we are trying to avoid prescriptive or stipulative definition. We prefer to offer a relatively naturalistic account based on the existing literature and on the talk of practising teachers - to many of whom we are grateful for discussions which have gone to inform what follows.