Every year between three and four hundred papers are published on the topic of insulin action. This extraordinary publication rate prevents any author from includ ing an exhaustive bibliography in any review or book. Perhaps due to this there is no single text that attempts to cover the effects and the mechanism of action of insulin. This book is such an attempt. I intend to present a review of the physiological effects of insulin, the pathology of defects in the action of insulin, and the current views on the mechanism of action of this hormone. I make no apology for the fact that the bibliography will not be extensive and that the amount of experimental detail and data discussed will be kept to a relevant minimum. This book is not intended for the expert in the field, but for the second- or third-year undergraduate and graduate student of medicine, biochemistry, physiology or related disciplines, and will be valuable as a reference source for research workers. The book is presented as a guide, a summary of the ideas and facts; it will present a reader with a foretaste of a fascinating and ever-changing field. I have attempted to be up-to-date with published research work. Any significant contributions to the field not included in the first draft have been added as footnotes. I assume a basic knowledge of the metabolic pathways of carbohydrates, fats and proteins.