Severe asthma remains poorly understood and frustrating to care for, partly because it is a heterogeneous disease. Patients with severe asthma disproportionately consume health care resources related to asthma. Severe asthma may develop over time, or shortly after onset of the disease. The genetic and environmental elements that may be most important in the development of severe disease are poorly understood, but likely include both allergic and nonallergic elements. Physiologically, these patients often have air trapping, airway collapsibility, and a high degree of methacholine hyperresponsiveness. Specific phenotypes of severe asthma are only beginning to be defined. However, describing severe asthma by age at onset (early- vs. late-onset) appears to describe two phenotypes that differ at immunologic, physiologic, epidemiologic, and pathologic levels. In particular, early-onset severe asthma is a more allergic-associated disease than late-onset severe asthma. In addition, patients with severe asthma can be defined on the basis of presence and type of inflammation. Severe asthma with persistent eosinophilia (of either early or late onset) is more symptomatic and has more near-fatal events. However, at least 50% of patients with severe asthma have very little identifiable inflammation. Thus, "steroid resistance" may occur at numerous levels, not all of which are caused by a lack of effect of steroids on inflammation. Treatment remains problematic, with corticosteroids remaining the most effective therapy. However, 5-lipoxygenase inhibitors, anti-IgE, and immunomodulatory drugs are also likely to have a place in treatment. Improving therapy in this disease will require a better understanding of the phenotypes involved.