Although type 2 diabetes and cancer are major health concerns among the adult population, few studies have directly addressed the relationship between the two, or the impact of diabetes on cancer outcomes. Diabetes and hyperglycemia are associated with an elevated risk of developing pancreatic, liver, colon, breast, and endometrial cancer. When treating cancer patients who have diabetes, clinicians must consider the cardiac, renal, and neurologic complications commonly associated with diabetes. Chemotherapeutic choices and, ultimately, the outcome for cancers may be affected by the avoidance of agents that have been shown to provide the best clinical response and survival in cancer patients without other disease complications. Evidence from population-based studies and clinical trials indicate that hyperglycemic and diabetic patients experience higher mortality and recurrence rates after diagnosis with, and treatment for, cancer. Evidence from the intensive care literature indicates that achieving glucose control leads to better clinical outcomes. If so, continued improvement of cancer outcomes may depend upon improved diabetes control. The association between diabetes and cancer is complex and warrants further study as the general population ages and the magnitude of both health problems continues to grow. Here we consider the influence of diabetes and hyperglycemia on the development, treatment, and long-term outcomes of cancer.