The core clinical feature of autism is a profound disturbance in the emergence of social relations, apparent as early as the very first months of life and almost always by age three years. Many different theories have been proposed to explain this dramatic developmental dysfunction, including cognitive, linguistic, arousal and, most recently, "theory of mind" hypotheses. There is great heterogeneity among autistic individuals and no single explanation captures all the clinical phenomena. Because of the divergent theories and their associated treatment approaches, parents are often burdened by conflicting advice. Field trials and other studies have provided excellent diagnostic criteria for autism for DSM-IV and ICD-10, with high sensitivity and specificity. Careful definition of the clinical phenotype is essential for neurobiological, genetic and behavioral research. While many lines of evidence point to underlying disturbances in brain maturation, no specific CNS dysfunction or biological correlate has been discovered. Rigorous research is not only essential for improving the understanding and treatment of autism; such studies may also help elucidate the normal preconditions for socialization and the pathways that allow a child to enter into the world of human relationships.