This book attempts to explore the contribution that biochemistry has made, thus far, to our understanding of the endocrine pancreas and its relationship to diabetes mellitus. It was written with the aim of using an important clinical problem to illustrate, to medical students, that there are many aspects of the biochemistry taught in the early years which have direct relevance to clinical medicine. Furthermore, it is hoped that such information might provide biochemistry students with a frame- work on which to base further studies. To this end a selection of recent references has been placed at the end of each chapter. In spite of considerable advances in our understanding of diabetes mellitus, it is still a disease which many physicians do not seem to com- prehend. This is in part related to their lack of understanding of the molecular biology of the disease. Advances in this area have been dramatic in recent years and we are now able to offer a molecular basis for a rational approach to therapy. It may be therefore that this book will provide some physicians with the information they require to help them gain a deeper understanding of the disease. I hope that everyone who reads this book is able to capture some of the fascination that the islets of Langerhans hold for myself and the many other workers actively engaged in trying to unravel their mys- teries.