Abstract Background: Increasing evidence suggests that diabetes increases the risk of developing different types of cancer. Hyperinsulinemia, hyperglycemia and chronic inflammation, characteristic of diabetes, could represent possible mechanisms involved in cancer development in diabetic patients. At the same time, cancer increases the risk of developing new-onset diabetes, mainly caused by the use of specific anticancer therapies. Of note, diabetes has been associated with a ∼10% increase in mortality for all cancers in comparison with subjects who did not have diabetes. Diabetes is associated with a worse prognosis in patients with cancer, and more recent findings suggest a key role for poor glycemic control in this regard. Nevertheless, the association between glycemic control and cancer outcomes in oncologic patients with diabetes remains unsettled and poorly debated. Purpose: The current review seeks to summarize the available evidence on the effect of glycemic control on cancer outcomes, as well as on the possibility that timely treatment of hyperglycemia and improved glycemic control in patients with cancer and diabetes may favorably affect cancer outcomes.