Asthma is a chronic disease associated with substantial morbidity, mortality, and health care use. Between 1980 and 1994, the self-reported prevalence of asthma increased 75% among all race, sex, and age groups in every region of the United States. Although an estimated 14.6 million persons had asthma in the United States in 1996, more recent studies have suggested a plateauing of the prevalence of the disease. Because establishing the diagnosis of asthma and characterizing the features of the disease have long been difficult for both the clinician and the researcher, studies determining the frequency of asthma across different countries and over time, seeking clues to the etiology of the disease, and monitoring for untoward variations provide the clinician with additional resources to manage patients with asthma. The purpose of this article is to provide a brief overview of the current and emerging national and international trends in the epidemiology of asthma.