Overweight and obesity is a growing “world-wide epidemic problem” as many as, because two-thirds of the adult population and a growing number of children are overweight. The prevalence of diabetes mellitus, especially type 2 diabetes mellitus, metabolic syndrome and hypertension are significantly increased with the prevalence of obesity. Many patients are both diabetic and hypertensive as well as being obese. Obesity, hypertension and diabetes are involved in the six leading causes of death in the United States. Obesity, hypertension, and diabetes (metabolic syndrome) are high risk factors for subsequent cardiovascular and renal complications. Sympathetic nervous activation and insulin resistance frequently observed in obesity may play major roles in cardiovascular and renal complications in patients with hypertension, diabetes, and obesity. The purpose of this article is to provide a synthesis of the current findings from epidemiological surveys on the relationships of obesity, hypertension and diabetes mellitus and the mechanisms of the onset and maintenance of cardiovascular and renal complications in obesity, hypertension, diabetes mellitus and metabolic syndrome. A better understanding of the close linkage of obesity with hypertension and diabetes may help with the clinical treatment of obesity, and may lead to reductions in cardiovascular and renal risk.