This chapter moves from the original form of attachment theory and research to a more comprehensive behavioral systems theory of personality, motivation, and social behavior that considers both individual differences and the impact on mental processes and behavior of relationship partners and other aspects of social situations. The chapter begins by explaining the behavioral systems construct, including its species-universal and individual-differences aspects, which interact with social situations to shape social behavior. The behavioral systems studied to date are discussed: attachment, exploration, caregiving, sex, and power. More emphasis is placed on the attachment system than on the other behavioral systems because it has received more research attention, but new research related to the other behavioral systems is summarized. Avenues are suggested for further theorizing and research that will lead to a more complete behavioral systems theory of personality and social behavior.