Astroviruses are small, round, nonenveloped, positive-sense single-stranded RNA viruses. They were named for their “star-like” appearance when visualized by negative staining electron microscopy in the feces of infants suffering from gastroenteritis. 1 , 2 Since their original discovery, astroviruses have been identified in almost all animal species examined. 3 In the vast majority of these species, astrovirus infection is transmitted through the fecal–oral route and causes acute gastroenteritis. In humans, the classical astroviruses are associated with mild to moderate diarrhea and are recognized as one of the leading causes of enteritis in children and the immunocompromised. 4 , 5 More recently, astroviruses have been associated with encephalitis. 6 , 7 In other species, especially poultry, diarrhea can be severe and can involve other organ systems including the liver and kidneys. 8 , 9 Historically, astroviruses have been thought to be species-specific, with only limited evidence of interspecies transmission. 10 However, there is increasing evidence that astroviruses can infect multiple species. Convincing evidence for this was first found for avian astroviruses, 11–17 but recent reports suggest the same may be true for mammalian astroviruses, 18–20 and that there may even be transmission between birds and mammals. 21 More recently, astrovirus genotypes associated with human infections were detected in nonhuman primates, strongly supporting the suggestion that astroviruses can cross species barriers. 21a