Oxytocin is a neuropeptide involved in a wide variety of social behaviors in diverse species. Recent research on its effects in humans has generated an arresting picture of its role in the dynamic function of the social brain. This review presents a broad overview of this uniquely social peptide, with a particular focus on extant studies of its effects in humans. After a short discussion of the evolutionary history of the oxytocin system, critical aspects of its peripheral and central physiology, and several salient technical issues surrounding human oxytocin research, a systematic review of studies of the effects of intranasal oxytocin in humans is presented. These effects include alterations in social decision making, processing of social stimuli, certain uniquely social behaviors (e.g., eye contact), and social memory. Oxytocin's prosocial influence is then framed by an evolutionary perspective on its role in mammalian social bonding and attachment. Finally, limitations in current human oxytocin research and oxytocin's potential therapeutic applications are discussed. Key conclusions are (1) human research with intranasal oxytocin has uniquely enhanced our understanding of the microstructure and function of the human social brain, and (2) the oxytocin system is a promising target for therapeutic interventions in a variety of conditions, especially those characterized by anxiety and aberrations in social function.