The dramatic increase in the worldwide prevalence of obesity has paralleled the increase in the prevalence of obstructive sleep apnea (OSA). Even with heightened awareness by the lay and medical communities, OSA is still markedly under-diagnosed, as evidenced by the persistent presentation of late-stage cardiovascular complications in obese individuals newly diagnosed with sleep apnea. The clinical sequela of untreated and poorly-treated sleep apnea include conditions that are considered components of the metabolic syndrome for which central obesity is one of the major case-defining features. Hence, in this review of obesity and sleep apnea, it is unavoidable to include discussion of sleep apnea and other components of the metabolic syndrome. Proponents of this clinical perspective suggest that there are mutual genetic determinants that give rise to common phenotypic features and allow clustering of sleep apnea with the other components of the metabolic syndrome. Perhaps, the strongest observational evidence to support a link between sleep apnea and obesity is the similarity in age distribution of symptomatic sleep apnea and metabolic syndrome. The putative causal links between sleep apnea and each individual component of the metabolic syndrome have been extensively evaluated and have implicated bidirectional causality in certain metabolic conditions, such as obesity and sleep apnea, sleep apnea and diabetes mellitus, and obesity and diabetes mellitus. These studies collectively suggest that even modest weight loss improves OSA, and positively affects both metabolic and cardiovascular risk profiles.