Agreeableness is a summary label for individual differences in the motivation to maintain positive relations with others. Agreeableness is one of the major dimensions in the Big Five structural model of personality. It is also a major domain in the Five Factor Model of personality. This chapter provides an overview of the considerable body of research concerning the conceptualization, assessment, and etiology of Agreeableness with a focus on its six facets. It concludes with a discussion of alternative theoretical explanations for Agreeableness. In particular, an opponent process model that involves two competing motive systems is applied to the processes underlying Agreeableness.