The insulin receptor belongs to the family of receptor tyrosine kinases. Over the past decade, there has been rapid progress in defining the structure of the insulin receptor and in elucidating the mechanisms whereby the receptor mediates the actions of insulin on the target cell. Furthermore, the insulin receptor gene is the target for mutations that cause genetic disease associated with insulin-resistant diabetes mellitus. In the future, application of the methods of physical biochemistry will further refine our understanding of the molecular mechanisms of insulin action. This greater understanding of the mechanisms of insulin action may promote the development of improved therapeutic approaches to treatment of non-insulin-dependent diabetes mellitus.