Obsessive-compulsive disorder (OCD) is a neuropsychiatric disease and a complex genetic illness. Genetic studies have demonstrated that both biological and environmental factors are important in the etiology of OCD. OCD family studies published since the 1930's provide strong evidence for increased recurrence rates among relatives of probands, reflecting an approximate four-fold increased risk compared to relatives of controls. Twin studies have further reported increased heritability estimates in child (45—65%) versus adult (27—47%) OCD samples. Attempts to narrow down the search for OCD vulnerability genes have included genome-wide linkage scans, reporting regions of interest for OCD on chromosomes 9p and 10p, and for compulsive hoarding on 3q and 14. More than 80 OCD candidate gene studies have been reported over the last decade, primarily selected for study due to their proximal location to linkage peaks (positional candidates) or their putative etiologic role in neurotransmitter pathways or anatomy related to OCD (functional candidates). Specifically, genes within serotonin, dopamine and glutamate pathways and those involved in white matter formation have been the center of focus. The glutamate transporter gene SLC1A1 is the only one of these that has been consistently associated across OCD samples, although not at genome-wide significance levels. The relatively small sample sizes and broad heterogeneity of samples have been limitations of OCD genetic studies to date. Most recently, a collaborative group of over 20 international research sites has undertaken a genome-wide association study (GWAS) of OCD, in further search of common single nucleotide polymorphism (SNP) markers, copy number variants (CNVs) and other rare genetic events that may contribute to its etiology.