Avian encephalomyelitis virus (AEV) is a picornavirus with a predilection for the central nervous system and other parenchymous organs of chickens that is transmitted by the oral-faecal route. The virus may be spread by the vertical and horizontal routes and, because of its great stability, contaminated areas may remain infectious for long periods. The egg-adapted Van Roekel strain is highly neurotropic and does not grow efficiently in the enteric tract of the chicken. Despite this, the virion polypeptides of both naturally-occurring strains and the Van Roekel strain are antigenically identical, and there is only one virus serotype. Natural infection by AEV induces a multi-organ disease with similarities in its pathogenesis for chickens to that of Coxsackie B3 in neonatal and infant mice. Field isolates of AEV can be adapted to grow in chicken embryo brain cultures, but virus growth is highly cell-associated. Simple reliable antibody assays to determine the presence of recent infections or to assess vaccine efficacy were not available until comparatively recently when antibody detection assays, many involving the use of chick embryos, were replaced by enzyme-linked immunosorbent assays. Effective vaccination programmes employing live, highly enterotropic strains of AEV, have greatly reduced the incidence of the disease.