Diabetes mellitus is the most common group of metabolic diseases and is characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. Most patients with diabetes are type 2 (90%); the remaining patients have type 1 disease. Athletes with diabetes range from the athlete participating in various youth sports to the competitive Olympic athlete and present a significant challenge to themselves and the medical staff who care for them on a daily basis. Each sport and the type of exercise have their own effects on diabetes management with numerous factors that significantly affect glucose levels, including stress, level of hydration, the rate of glycogenolysis and gluconeogenesis, and the secretion of counter-regulatory hormones. This article provides a general overview of diabetes mellitus, the effects of exercise on glucose levels, and a detailed review of the potential complications encountered in the management of diabetes in the athlete.