A major challenge confronting the pharmaceutical scientist working with protein formulation is the instability during processing, storage and use of the protein drug. This book reviews stability aspects encountered since the pioneering isolation of insulin in 1921. An introductory chapter, treating insulin purity and stability in historical perspective, is followed by a chapter with a description of the structure of insulin. The main part of the book is a comprehensive review of the literature dealing with stability of insulin in pharmaceutical formulation, including biological stability and the physical and chemical decomposition of insulin. The physical stability (i.e. the tendency of insulin to form insoluble fibrils) is reviewed as are methods to stabilize insulin for long-term use in infusion pumps. The book will be of interest to research and clinical pharmacologists involved with insulin and other protein-based drug substances.