Oxytocin is a neuropeptide that regulates physiology and behaviors associated with reproduction as well as other social and nonsocial behaviors. The actions of oxytocin are mediated by a single G protein-coupled receptor distributed in many peripheral tissues and specific brain areas. Oxytocin secreted into the circulation stimulates uterine contractions during labor and stimulates milk ejection during lactation. Oxytocin released within the brain modulates social recognition, sexual behavior, maternal care, and maternal bonding as well as pair bonding in monogamous species. Appetite, mood, and anxiety are also affected by oxytocin. Paralleling its effects on social behavior in animals, oxytocin has been implicated in facilitating trust, in the processing of social information, and in intuiting emotional states of others. It has also been suggested that it plays a role in etiology, and may represent a potential treatment, of social deficits in psychiatric disorders such as autism spectrum disorder. Most recently, specific genetic and epigenetic changes in the oxytocin receptor gene have been correlated with specific behavioral traits, including autism. In this chapter we review the oxytocin system and its role in regulating physiology and behavior.