Diabetes mellitus is a disorder of endocrine function that affects metabolic and circulatory mechanisms. As a chronic disease, diabetes mellitus is characterized by glucose intolerance and distortions in fat metabolism, caused by either relative or absolute insulin deficiency. These two kinds of insulin deficiency are used to categorize the two primary variants of diabetes: Type 1, formerly called insulin-dependent diabetes mellitus (IDDM), and Type 2, formerly called non-insulin-dependent diabetes mellitus (NIDDM). This chapter focuses on Type 2 disease, and briefly covers Type 1 disease as well. Type 1 diabetes mellitus (T1DM) results from the absence of insulin secretion from the pancreatic beta-cells after beta-cell destruction by glutamic acid decarboxylase antibodies. Type 2 diabetes mellitus (T2DM) is a polygenic disease characterized first by insulin resistance, leading to compensatory hyperinsulinemia. Hyperinsulinemia also leads to blood vessel proliferation and, concomitantly, damage to vessel intimae.