Obesity, hypertension, obesity-related hypertension and the metabolic syndrome are growing health problems. Importantly, while frequently associated, not all obese subjects have hypertension. It is widely recognized that heightened sympathetic nerve activity and insulin resistance (or hyperinsulinemia) relate to obesity, hypertension and the metabolic syndrome. However, the precise relationships between heightened sympathetic nerve activity and insulin resistance in obesity and hypertension remain uncertain. Many investigators have indicated that insulin resistance may play a major role in obesity, hypertension, and obesity-related hypertension, and heightened sympathetic nerve activity accompanies insulin resistance. On the other hand, some investigators have reported in a series of longitudinal studies that heightened sympathetic nerve activity may be a prime mover for obesity, hypertension, and obesity-related hypertension, and insulin resistance may play an ancillary role. Blunted sympathetic nerve responses to insulin stimulation was observed during oral glucose loading only in insulin resistant subjects, obese subjects or hypertensive subjects, suggesting that the relationships between insulin resistance and sympathetic nerve activity might be different due to the stage of obesity, hypertension, or severity of insulin resistance. Both hypotheses emphasize the close linkage between insulin resistance and heightened sympathetic nerve activity; however, which is the prime mover for obesity or hypertension remains unknown. The purpose of this review is to provide the current findings on the relationships between sympathetic nerve activity and insulin resistance in obesity, hypertension, and the metabolic syndrome.