Obsessive compulsive disorder (OCD) is a common psychiatric disorder that can have disabling effects on both adults and children. Twin, family, segregation, and linkage studies have demonstrated that OCD is familial, that the familiality is due in part to genetic factors and there are regions of the genome which very likely harbor susceptibility loci for OCD. Over 60 candidate gene studies have been conducted. Most studies have focused on genes in the serotonergic and dopaminergic pathways. Unfortunately, none have achieved genome‐wide significance and with the exception of the glutamate transporter gene, none have been reliably replicated. Future research will requite much larger samples and the collaboration of researchers to be able to identify susceptibility loci for OCD. © 2008 Wiley‐Liss, Inc.