First published in 2001. This is Volume 2 in a series on the science of mental health and focuses on Autism. It begins with an overview presentation of a case and a recent consensus description of diagnostic instruments for autistic-spectrum disorders. This is followed by recent epidemiologic data. Epidemiology has an enhanced prominence in studies of autism because there is an unresolved controversy about the incidence of new cases of autism. In some surveys the incidence of autism appears to be increasing, and the controversy is over whether this is a true observation or whether greater diagnostic awareness is leading to the apparent increase. This volume attempt to resolve the conflicting data as if autism is indeed on the rise, there must be a new, significant environmental risk factor, which has to be identified with a view to prevention.