There is ample evidence that the sympathetic nervous system is important in the etiology of essential hypertension. Plasma catecholamines such as norepinephrine and epinephrine are the most common indexes of sympathetic function used in studies of essential hypertension. Plasma norepinephrine is higher in young essential hypertensive patients than in normotensive subjects. Other methods to examine sympathetic activity, such as blood pressure response to sympatholytic agents, measurement of regional sympathetic activity, vascular reactivity to sympathetic agonists, power spectral analysis, and microneurography, have all provided further evidence for enhanced sympathetic activity in essential hypertension, especially in younger subjects. Certain groups that make up a substantial part of the essential hypertensive population, such as obese subjects, have heightened sympathetic activity that could contribute to hypertension. Plasma norepinephrine levels are significantly higher in obese compared with nonobese subjects, and the remarkable fall in blood pressure with weight loss in obese subjects is correlated with reductions in plasma norepinephrine. Antihypertensive agents have variable effects on sympathetic activity; some agents (diuretics and direct vasodilators) have elevating effects, some agents (centrally acting agents and alpha-antagonists) have lowering effects, and others (converting enzyme inhibitors, calcium blockers, and beta-blockers) have mixed effects. Tailoring therapy toward agents that reduce sympathetic activity for specific groups perceived as having neurogenic hypertension, such as obese subjects, is a goal yet to be attained.