Asthma is more prevalent in obese, compared with normal weight, subjects. Our aim has been to review current knowledge of the impact of obesity on asthma severity, asthma control, and response to therapy. Several studies have shown that overweight and obesity is associated with more severe asthma and impaired quality of life, compared with normal weight individuals. Furthermore, obesity is associated with poorer asthma control, as assessed by asthma control questionnaires, limitations in daily activities, breathlessness and wheezing, use of rescue medication, unscheduled doctor visits, emergency department visits, and hospitalizations for acute asthma. Studies of the impact of a high body mass index on response to asthma therapy have, however, revealed conflicting results. Most studies show that overweight and obesity is associated with less favorable response to asthma therapy, with regard to symptoms, level of FEV1, fraction of exhaled nitric oxide, and airway responsiveness. Some studies suggest that asthma in the obese patient might be more responsive to leukotriene modifiers, orchestrated by leptin and/or adiponectin derived from adipose tissue, than to inhaled corticosteroids, possibly reflecting differences in the underlying airway inflammation in obese versus non-obese asthmatics. In conclusion, overweight and obesity is associated with poorer asthma control and, very importantly, overall poorer response to asthma therapy, compared with normal weight individuals.