Asthma, an inflammatory disease of the airways, is a common condition, currently affecting about 9% of adults. Obesity is another disease with high prevalence in adults, and asthma that develops in obese individuals appears to be distinct from other forms of asthma, in being particularly severe and difficult to control even with oral corticosteroids. Moreover, recent studies suggest that obesity may have a causal relationship with asthma. In this review, we discuss possible obesity-driven metabolic and immunological pathways that might lead to asthma that is associated with obesity. Our understanding of the mechanisms that underlie this association will likely lead in the future to improved treatments for this significant unmet medical need.