Transition metal oxides form a series of compounds with a uniquely wide range of electronic properties. They have important applications as dielectrics, semiconductors and metals, and as materials for magnetic and optical uses. The discovery of high temperature superconductors has brought the attention of a wide scientific community to this area and has highlighted the problems involved in trying to understand transition metal oxides. The present book is not primarily about Tc superconductors, although their main properties are discussed in the final sections. The main aim is to describe the varied electronic behaviour shown by transition metal oxides, and to discuss the different types of theoretical models that have been proposed to interpret this behaviour.