Diabetes mellitus is a growing epidemic with a prevalence among patients with heart failure (HF) approaching 30%. Diabetes worsens the prognosis of HF, and the pathophysiology is complex and multifactorial. Early detection of subtle alterations in cardiac function by modern tools, such as Doppler echocardiography or brain natriuretic peptide dosage, is thus important in these patients. All drugs known to be effective in HF with systolic dysfunction are also effective in patients with diabetes. Angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists also seem particularly useful. Overall, however, little is known about the treatment of diabetic patients with HF, especially in case of preserved systolic function. Ongoing and future trials should help to determine the best treatment for these patients with or without associated diabetes. This review assesses the relationships between diabetes mellitus and HF and discusses the various medical strategies.