Lung function in a healthy individual varies in a circadian rhythm, with peak lung function occurring near 4:00 pm (1600 hours) and minimal lung function occurring near 4:00 am (0400 hours). An episode of nocturnal asthma is characterized by an exaggeration in this normal variation in lung function from daytime to nighttime, with diurnal changes in pulmonary function generally of > 15%. The occurrence of nocturnal asthma is associated with increased morbidity and inadequate asthma control, and has an important negative impact on quality of life (QOL). Newer data have shed light on physiologic and immunologic mechanisms that underlie the nocturnal development of airway obstruction. It remains controversial whether nocturnal asthma is a distinct entity or is a manifestation of more severe asthma. The current data do not resolve these two alternatives, as well-controlled studies have reached opposite conclusions. However, the clinical associations of gastroesophageal reflux disease and obesity appear to be strong. The treatment of asthma with effective controller agents can reduce nighttime symptoms, improve psychometric outcomes, and improve QOL.