Significant advances have been made in characterizing the phenomenology and psychobiology of obsessive-compulsive disorder (OCD) in recent years. In many ways, such advances suggest a conceptualization of OCD as a relatively homogeneous neuropsychiatric entity, underpinned by particular mechanisms that manifest in universal symptoms. Nevertheless, some data have pointed to the heterogeneity of this disorder. A computerized literature search (MEDLINE: 1964-2001) was used to collect studies addressing the heterogeneity of OCD. In addition, reviews of the phenomenology, psychobiology, family studies, and treatment of OCD were examined in an attempt to collate data addressing this issue. There is a growing consensus that some subtypes of OCD are valid and provide a useful means of integrating data on its symptomatology, neurobiology, and treatment response; for example, OCD with comorbid tics is characterized by earlier onset, a particular range of OCD symptoms, and worse response to selective serotonin reuptake inhibitors. The heterogeneity of OCD has important clinical and research implications.