Diabetes mellitus refers to a group of common metabolic disorders that share the phenotype of hyperglycemia. Complications due to type 2 diabetes mellitus (T2DM) can be microvascular (ophthalmic, renal and neurologic involvement) or macrovascular (coronary, cerebrovascular and peripheral vascular involvement). Dyslipidemia, albuminuria and hypertension also play important roles in the macrovascular complications of DM. The pathophysiology of vascular disease in diabetic patients involves abnormalities of endothelial cells and vascular smooth muscle cells. This chapter covers the segmental and multivessel involvement and obstructive CAD, symptomatology, coronary artery calcification, plaque characteristics and distribution in the coronary arterial tree, morbidity, mortality and major adverse cardiac events, and treatment.