Diabetes mellitus (DM) is highly prevalent and is an important risk factor for congestive heart failure (HF). Increased left ventricular (LV) diastolic stiffness is recognized as the earliest manifestation of DM-induced LV dysfunction, but its pathophysiology remains incompletely understood. Mechanisms whereby DM increases LV diastolic stiffness differ between HF with normal LV ejection fraction (EF) (HFNEF) and HF with reduced LVEF (HFREF). In diabetic HFREF, fibrosis and deposition of advanced glycation end products (AGEs) are the most important contributors to high LV diastolic stiffness, whereas in diabetic HFNEF, elevated resting tension of hypertrophied cardiomyocytes is the most important contributor to high LV diastolic stiffness. As HF mortality remains high in DM despite proven efficacy of current treatments, better understanding of the pathophysiology of high LV diastolic stiffness could be beneficial for novel therapeutic strategies.