In the last years avian adenoviruses have caused diseases with similar symptoms and high losses in four countries. The causative viruses could be grouped by cross-neutralization tests and restriction enzyme analysis into four serotypes. Within the serotypes there exist strains with pathogenic properties as well as strains without pathogenicity. Epidemiologic investigations demonstrated that not only immunosuppression or co-infections produce diseases after adenovirus infections. Vertical transmission of the virus from parent stocks and deficiency of specific maternal antibodies in the chickens contribute to the development of diseases. Parent stocks, which were infected with different serotypes of adenovirus during their rearing period, produce chickens with maternal antibodies. These chickens are protected against early infections with avian adenoviruses.