Diabetes mellitus (diabetes Greek, pass through/siphon; mellitus Latin, honey sweet) is a heterogeneous, multifactorial metabolic disorder characterized by chronic hyperglycemia. This is often due to insulin resistance (type 2 diabetes) or an absolute insulin deficiency (type 1 diabetes). Diabetes is defined by elevated blood glucose concentrations that can be measured at the fasting state or during an oral glucose tolerance test or, alternatively, by measuring the degree of glycosylation of hemoglobin. Comorbidites, such as atherosclerotic cardiovascular disease, chronic kidney disease, and heart failure are briefly introduced and treatment options are described. An important aspect of diabetes therapy is diabetes self-management education and support (DSMES), and therapeutic options must be chosen based on the individual patient’s social and educational environment as well as comorbidities and response to treatment.