Although we know much more now than we did 50 years ago about autism, the nature, origin and even the definition of the condition are still debated and remain largely unknown. This special issue begins with a review of the facts about autistic disorders, as they are known at present. In their introduction, Elizabeth Hill & Uta Frith (2003) remind the reader that autism is no longer regarded as a rare disease. They provide examples of genetic and brain research that targets the biological causes of autism and they review the three major cognitive theories that are currently used to explain the core signs and symptoms of autism. Much more is known now about autism than was known only a few years ago, and there is justified hope that our understanding of autism will continue to accelerate at a fast pace. This issue contains examples of the cutting edge of research and highlights some of the most burning questions. Some of these relate to the diagnosis of Asperger syndrome (AS), the identification of subgroups in the autism spectrum and early signs of autistic disorder. Other questions relate to the brain abnormalities that underlie the putative cognitive deficits and whether these can be made visible through magnetic resonance imaging. The shared assumption among the contributors is that autism is a neurodevelopmental disorder that gives us a unique window on the relationship between mind and brain. The research reported elaborates the key theories that have been put forward to explain the signs and symptoms of autism. These theories try to explain the selective impact of brain abnormality on some of the most high–level mental functions, such as social insight, empathy and information processing style.