There is a global epidemic of asthma and obesity that is concentrated in Westernized and developed countries. A causal association in some people with asthma is suggested by observations that obesity precedes the onset of asthma and that bariatric surgery for morbid obesity can resolve asthma. The obese asthma phenotype features poor asthma control, limited response to corticosteroids, and an exaggeration of the physiological effects of obesity on lung function, which includes a reduction in expiratory reserve volume and airway closure occurring during tidal breathing. Obesity has important implications for asthma treatment. Increasing corticosteroid doses based on poor asthma control, as currently recommended in guidelines, may lead to overtreatment with corticosteroids in obese asthma. Enhanced bronchodilation, particularly of the small airways, may reduce the component of airway closure due to increased bronchomotor tone and suggests that greater emphasis should be placed on long-acting bronchodilators in obese asthma. The societal implications of this are important: with increasing obesity there will be increasing asthma from obesity, and the need to identify successful individual and societal weight-control strategies becomes a key goal.