Severe asthma is poorly understood clinically, physiologically, and pathologically. While milder forms of asthma are generally easily treated, more severe forms often remain refractory to the best current medical care. Although some patients with severe asthma have had severe disease for most of their lives, there appears to be a second group that develops severe disease in adulthood. Additionally, it is not clear which genetic and environmental elements may be the most important in the development of severe disease. Physiologically, these patients often have airtrapping and may have loss of elastic recoil, as well. The pathology demonstrates a heterogeneity of findings, including continued eosinophilic inflammation, structural changes, distal disease, and, in at least one third of patients, a different pathology. Treatment remains problematic and likely will remain so until a better understanding of this disease develops.