Cardiovascular disease is a leading cause of death and disability in patients with diabetes or metabolic syndrome (MS). The available data suggest that many patients with diabetes or MS already have vascular abnormalities by the time they are diagnosed with their metabolic disorder. Endothelial dysfunction (ED), which is one of the initial steps in the process of vascular disease, is often present in patients with diabetes or MS. Although the precise mechanism(s) by which diabetes or MS causes ED remains to be elucidated, several possibilities exist. Hyperglycemia, hyperinsulinemia, increased oxidative stress, and diabetic dyslipidemia can all contribute to ED individually or in concert with one another. ED in the setting of diabetes or MS can subsequently result in the activation of a variety of pathways that alter vascular function and participate in the process of vascular remodeling and atherosclerosis. Because insulin resistance is the predominant mechanism responsible for various perturbations seen in MS or diabetes, it is essential to develop a therapeutic strategy that can improve insulin sensitivity with the hope that such interventions would reduce the risk of future cardiovascular events.