Severe asthma represents less than 10% of all asthma, but these patients are responsible for a disproportionate share of the health care costs and morbidity associated with the disease. A significant challenge in the diagnosis and management of severe asthma is the ability to identify accurately the patients most at risk for adverse outcomes, such as medication side effects, emergency department visits, hospitalization, near-fatal events, or disability from persistent symptoms or chronic lung function abnormalities. To improve the treatment of these patients, we must improve our understanding of the mechanisms responsible for severe disease. To achieve this goal, it is imperative to develop a common definition of severe asthma to allow adequate characterization of the disease clinically and provide the opportunity to compare results from many studies. Several severe asthma phenotypes have been described in the literature on the basis of the age of patients, age of disease onset, corticosteroid resistance, chronic airflow obstruction, and evidence for eosinophilic airway inflammation on biopsy. These phenotypes have led to an emerging interest in the use of noninvasive methods to monitor airway inflammation in severe asthma. Treatment algorithms based on markers of airway inflammation may decrease measures of health care utilization in severe asthma.