Severe asthma remains poorly understood and frustrating to treat, partly because it is a heterogeneous disease. Recent improvements in the definition of severe asthma have allowed better characterization of the phenotypes of severe asthma and the related physiologic and pathologic abnormalities. Early-onset severe asthma is a more allergy-associated disease than late-onset asthma. Persistent eosinophilia is more commonly seen in patients who have late-onset disease but is associated with a more symptomatic disease in both early- and late-onset disease. Recent studies suggest that response to therapy in severe asthma may depend on the phenotype.