The prevalences of asthma and obesity have increased substantially in recent decades in many countries, leading to speculation that obese persons might be at increased risk of asthma development. In adults cross-sectional, case-control, prospective, and weight-loss studies are in the aggregate consistent with a role for obesity in the pathogenesis of asthma. In children 3 of 4 prospective studies also show a significant association between excess weight and asthma incidence. Because of the methodologic shortcomings of many studies, these findings are inconclusive, however. Population surveys do suggest that persons with asthma are disproportionately obese compared with persons who have never had asthma. Weight-loss studies on the basis of behavioral change and bariatric studies have shown substantial improvements in the clinical status of many obese patients with asthma who lost weight. Clarifying the nature of the relationship between obesity and asthma incidence and the role of weight management among patients with asthma are both critical areas with important ramifications for the prevention and treatment of asthma.