The purpose of this article is to provide a general update of diabetes with a focus on newer developments that may impact risk assessment. The main reasons for concern about diabetes are the associated microvascular and macrovascular complications. Control of blood glucose levels, primarily assessed by measurement of hemoglobin A1c, has been shown to reduce the occurrence of microvascular complications. In conjunction with control of other risk factors, control of blood glucose levels reduces the occurrence of macrovascular complications. Type 1 diabetes is now known to be an autoimmune disease. For unknown reasons, the patient's immune system destroys its own pancreatic insulin-producing beta cells. The hallmark of type 2 diabetes is insulin resistance, which may be mediated by one of a number of gene defects. At some point, the pancreatic beta cells are unable to compensate for the insulin resistance by increasing insulin secretion. Type 2 diabetes then appears. Prediabetes, encompassing impaired fasting glucose and impaired glucose tolerance, is an important clinical entity that precedes the diagnosis of diabetes in many cases. Diet and exercise are the most important interventions for reducing the progression of insulin resistance to diabetes. Once diabetes develops, medications are matched to pathophysiology.