Theorists have long noted that people’s moral circles have expanded over the course of history, with modern people extending moral concern to entities—both human and nonhuman—that our ancestors would never have considered including within their moral boundaries. In recent decades, researchers have sought a comprehensive understanding of the psychology of moral expansiveness. We first review the history of conceptual and methodological approaches in understanding our moral boundaries, with a particular focus on the recently developed Moral Expansiveness Scale. We then explore individual differences in moral expansiveness, attributes of entities that predict their inclusion in moral circles, and cognitive and motivational factors that help explain what we include within our moral boundaries and why they may shrink or expand. Throughout, we highlight the consequences of these psychological effects for real-world ethical decision making.