Our understanding of the epidemiology of astrovirus-associated gastroenteritis has changed markedly with each improvement in detection method. In early surveys based on electronmicroscopy (EM), astroviruses appeared to be a rare cause of gastroenteritis, being found in fewer than 1% of children with diarrhea, usually in small outbreaks of disease and primarily during the winter season. The development and use of monoclonal antibodies and enzyme immunoassays (EIA) to detect astroviruses led to reports of a higher prevalence (2.5%-9%) of astrovirus infection among patients hospitalized with diarrhea. Astroviruses appeared second only to rotaviruses as a cause of hospitalization for childhood viral gastroenteritis. Studies based on EIA detection of astroviruses indicate that astroviruses are common causes of diarrhea in children worldwide, and that most children are infected during their first two years of life. The elderly and the immunocompromised represent high-risk groups as well. The observations that newborns monitored prospectively rarely have repeat disease and that the rate of detection decreases with increasing age suggest that immunity to astroviruses, as immunity to rotaviruses, may develop early in life. The cloning and sequencing of astroviruses have led to more sensitive assays to detect the viruses by reverse transcription, polymerase chain reaction (RT-PCR). Application of RT-PCR for detection of astroviruses in children in day-care centers showed a marked increase in the detected prevalence of astrovirus-associated diarrhea, the rate of asymptomatic infection, and the duration of shedding of virus among those infected, when compared with studies that used other methods. As with rotaviruses, neither the mode of transmission nor the reservoir of astrovirus infection has been identified. Both immune and molecular-based assays to detect astrovirus serotypes indicate that serotype 1 is most common worldwide, although the predominant serotypes may vary by region and time. In the absence of obvious strategies to prevent astrovirus-associated diarrhea, vaccines might be considered if further studies establish that the disease burden would render such a vaccine cost-effective.