The epidemiology of asthma is complex but essential in enhancing the understanding of a disease that affects millions of patients. Asthma is associated with significant morbidity and mortality. Asthma prevalence rates in the United States reached a plateau after 1998 with an estimated overall prevalence of 3.8% in 2003. Racial disparities exist and there are staggering differences in morbidity and mortality. The analysis of data collected from epidemiologic studies continues to be a critical part of enhancing the understanding of the pathophysiology of asthma, which will lead to improved patient outcomes.