The core model, K, is a generalization of Gödel's constructible universe of set theory; K is used to produce 'fine structural' results of a less restrictive kind. This book aims to introduce the core model to those with a basic knowledge of axiomatic set theory. The covering lemma for K is the main technical result but other applications are also considered. The author gives a full exposition of general fine structure and of iterated ultrapowers and concludes the work with a short section on the difficulties encountered in constructing more general core models using 'extenders'.