Technical Abstract: Avian astroviruses comprise a diverse group of viruses affecting many avian species and causing enteritis, hepatitis and nephritis. To date, six different astroviruses have been identified in avian species based on the species of origin and viral genome characteristics: two turkey-origin astroviruses [Turkey Astrovirus type 1 (TAstV-1) and type 2 (TAstV-2)]; two chicken-origin astroviruses: [Avian nephritis virus (ANV) and Chicken Astrovirus (CAstV)]; and two duck-origin astrovirus [Duck Astrovirus type 1 (DAstV-1) and type 2 (DAstV-2)]. As in mammals, astroviruses are commonly associated with enteritis in avian species. Enteric diseases cause substantial economic losses in commercial poultry, and many enteric disease syndromes have been described including runting-stunting syndrome of broilers (RSS), poult enteritis complex or syndrome (PEC or PES), and poult enteritis mortality syndrome (PEMS). Astroviruses are among the most common viruses found in cases of PEC, PES, and PEMS in turkeys. Astroviruses have also been isolated from cases of RSS in broilers and have been previously associated with poor weight gain, enteric disease and kidney disease in chickens, as well as in guinea fowl suffering from enteritis. The molecular characterization of the different avian astroviruses shows great genetic variability among each type, and this variability influences the ability to detect these viruses by molecular and serological techniques. Concomitant infections with different strains have been reported for avian astroviruses and this can result in recombination between strains. Recombination is likely to provide an important mechanism by which the sequence diversity of avian astroviruses is increased. In this chapter the different aspects related to avian astroviruses, including molecular biology, pathogenesis, diagnosis and control, are reviewed.