Obsessive Compulsive Disorder has fascinated clinicians for hundreds of years, but because of the high proportion (30-40%) of cases with childhood onset, the disorder holds particular interest for child psychiatrists. Moreover, unlike depression and schizophrenia, the disorder appears in virtually identical form in children to that in adults. New information derived from epidemiological, pharmacological and clinical descriptive studies, from studies of related disorders, and from a prospective study of 30 children with severe primary obsessive compulsive disorder, suggest that the disorder is considerably more common than had been thought and reaffirm intriguing neurological links, and severe morbidity of this condition if untreated.