Although much is known about humans’ responses to inequity, little is known about similar responses in other species. The goal of these issues is to bring together researchers focusing on both humans and non-humans to provide a synthesis of our knowledge of non-human responses to inequity to date, and what these data tell us about the evolution of humans’ responses. In this Introduction, I provide a brief background, highlighting both areas in which differences among the related literatures emerge and the ways in which the comparative approach can provide insight in to this question. As becomes clear in these issues, we have reached the point where we can move beyond documenting these responses in other species and develop research programs combining both human and non-human perspectives to better understand the evolution of fairness and justice.