Recent estimates of 0.7% for prevalence can be compared with British studies in the mid-70s where a combined rate for classical autism and the triad of impairments was 0.2%. Changing diagnostic criteria, broadening of the autism concept, diagnostic substitution (from ‘mental retardation’ to autism), improved services and awareness all contributed. Autism has a strong genetic basis but the possibility of additional causal environmental risk factors remains. The neuropathology and neurobiology point towards prenatal abnormal brain development. If environmental risks contribute to the increase in incidence, their impact must occur at or shortly after conception but no solid clues are yet available.