The isolation and purification of insulin nearly 100 years ago has been one of the most ground-breaking discoveries in the history of medicine. Subsequent determination of the specific structure of human insulin has permitted further developments and modifications of the formulations of insulin to allow improved quality of care and quality of life for patients with diabetes. In this paper, we will review insulin structure and biosynthesis, treatment and prognosis of type 1 diabetes prior to insulin therapy, experimentation leading to the discovery of insulin, and the first patients to be treated with insulin.