This chapter reviews the investigations that have indicated likely contributions of chromosomal risk regions, specific candidate genes and gene pathway networks to obsessive-compulsive disorder (OCD) etiology. As with other genetically complex medical disorders, methodological approaches in psychiatric genetics are shifting to genome-wide association studies (GWAS), which make more comprehensive single nucleotide polymorphism (SNP)-based assessments possible. Numerous gene products seem highly relevant to neurotransmitter system pathways and developmental sequences important in OCD, but only relatively few have been investigated. These include glutamate, dopamine, serotonin, and other systems, neurotrophic factor genes and their affiliated receptors, and genes indirectly implicated via comorbid disorders or suggested from animal models of OCD and OCD-related behaviors. Many models of behavioral changes resembling OCD-related features such as perseveration, compulsive grooming, food-restriction-induced compulsive wheel running, or drinking have been reported. OCD seems likely to be genotypically and phenotypically heterogeneous.