Acute severe asthma, formerly known as status asthmaticus, is defined as severe asthma unresponsive to repeated courses of beta-agonist therapy. It is a medical emergency that requires immediate recognition and treatment. Albuterol in combination with ipratropium bromide in the emergency department (ED) has been shown to decrease the time spent in the ED and the hospitalization rates. The benefits of ipratropium are not sustained after admission to the hospital. Oral or parenteral corticosteroids should be administered to all patients with acute severe asthma as early as possible because clinical benefits may not occur for a minimum of 6 to 12 hours. Viral respiratory infections are a common trigger for acute asthma; other causes include medical nonadherence, allergen exposure (especially pets and mold [e.g., Alternaria species]) in individuals who are severely atopic, nonsteroidal anti-inflammatory exposure in patients with aspirin allergy, irritant inhalation (e.g., smoke, paint), exercise, and insufficient use of inhaled or oral corticosteroids. The patient's history should focus on the acute assessment of asthma control and morbidity, including current use of oral or inhaled corticosteroids; the number of hospitalizations, ED visits, intensive care unit admissions, and intubations; the frequency of albuterol use; the presence of nighttime symptoms; activity intolerance; current medications; exposure to allergens; and other significant medical conditions. Severe airflow obstruction may be predicted by accessory muscle use, difficulty speaking, refusal to recline < 30°, a pulse of >120 beats/min, and decreased breath sounds. More objective measures of airway obstruction via peak flow or forced expiratory volume in 1 second and pulse oximetry before oxygen administration usually are helpful. Pulse oximetry values of >90% are reassuring, although CO₂ retention and a low partial pressure of oxygen may be missed.