Children, adolescents, and adults differ in many ways, including their approach to handling interpersonal situations and their tendencies to show concern for others feelings. Such interpersonal manifestations - including empathy, compassion, altruism, and love - are inherently interesting to and consequential for everyone. The current chapter focuses on the personality trait of agreeableness, including how this trait manifests in children, adolescents, and adults, and is related to behavioral outcomes across the lifespan. In addition to being intrinsically interesting and consequential, agreeableness is multifaceted. It intersects with self-regulation, including negative self-regulation (e.g., aggression, anger, hostility) and self-discipline and order (e.g., agreeable compliance and cooperation). Moreover, developmental research suggests that these distinct aspects of agreeableness are highly overlapping early in the lifespan (e.g., toddlerhood and early childhood), and become increasingly distinct from one another over the course of middle childhood and adolescence. Notable, these time periods are also when major social and academic changes are taking place, as well as emotional, cognitive, and biological changes within the individual, offering many possible mechanisms to potentially explain such changes.