Over the last two decades, the convergence of secular trends indicating increases in the prevalence of obesity and asthma has led to a hypothesis that these two disorders might be related. Although the mechanisms underlying a putative relationship between obesity and asthma have not been fully described, a relatively mature body of literature suggests that obesity increases the risk of incident asthma. This article addresses studies that could be interpreted as supporting the hypothesis that obesity leads to asthma. We evaluate animal studies that provide biological underpinnings to an association between the two disorders and clinical and epidemiologic studies that suggest that the relationship between these two disorders is clinically important.