Epidemiological data indicate that obesity is a risk factor for incident asthma, and that obesity is also associated with increased asthma severity. Importantly, obesity antedates asthma. The observations that weight loss improves asthma and that obese mice have innate airway hyperresponsiveness and increased responses to asthma triggers also support a relationship between obesity and asthma. The basis for this relationship is unknown, but might be the result of common etiologies, comorbidities, effects of obesity on lung volume or adipokines. Understanding the mechanistic basis for the relationship between obesity and asthma could lead to new therapeutic strategies for treatment of this susceptible population.