This chapter uncovers the meaning of altruism. First, an understanding of the essence of altruism is garnered from the consideration of a wide range of scholarship from different disciplines. Certain common themes of the concept are identified. Second, the connection between altruism and cosmopolitanism is revealed in an effort to show how altruistic behaviour between people from different countries can develop. Third, the influence of altruism in legal systems at the domestic level is considered by having reference to major studies that have analysed manifestations of altruism in European and US private law regimes. Other references to altruism or related concepts in international legal scholarship are also explored.