Type 2 diabetes mellitus (T2DM) is a heterogeneous group of metabolic diseases resulting from defects in insulin secretion, insulin action, or both. It comprises most diagnosed cases of diabetes and disproportionately affects minorities. This article describes the classification of diabetes mellitus; defines the metabolic syndrome; and discusses the diagnosis, risk factors, and screening criteria for T2DM. The pathophysiology of T2DM, its acute and chronic complications, and signs and symptoms are presented, along with a brief overview of treatment.