Over the past half-century attachment theory has emerged as a driving force for research and practice across much of the developmental psychology community. Some of the most compelling empirical support for the biological basis of attachment theory has come from research with monkeys. This chapter highlights some contributions that my colleagues and I have made in generating this evidence by broadening the range of activities associated with attachment throughout development, exploring the relationship between behavioral patterns and a host of biological processes including some genetic factors, and characterizing the consequences of different expressions of attachment for the external social world that is progressively engaged throughout development. This body of nonhuman primate research has featured several important turning points, largely the product of certain unique combinations of colleagues and circumstances that served to generate major inflection points in shaping the direction and scope of the research described here.