The literature confirms a well-established relationship between obesity and hypertension. However, the nature and strength of this association is inconsistent between certain racial/ethnic groups and between sexes; it also differs according to body distribution and at the extreme ranges of body mass and blood pressure. Similarly, the mechanism underlying the association of body mass to blood pressure is unclear, with several potential pathophysiologic mechanisms accounting, in part, for this relationship. Increased sympathetic nervous system activity, activation of the renin-angiotensin system, hyperleptinemia, insulin resistance, hemodynamic changes, inflammation, and endothelial dysfunction and oxidative stress have all been implicated and appear somewhat interconnected. Yet there is no evidence that one mechanism underlies the complement of physiologic derangements. This review highlights key epidemiologic studies and provides an overview of seven pathophysiologic mechanisms that account for the often-described but little understood association between body mass and blood pressure.