Demographic trends reveal that modern societies have become increasingly diverse. Within the social sciences, these changes have been reflected in concerns about the implications of social diversity. Although early research noted that diversity may have negative consequences for societies and individuals, more recent scholarship has indicated that diversity is not always translated into negative outcomes. These inconsistent findings initiated a scholarly debate concerning the impact of many different forms of diversity for a host of social outcomes. It is now clear that the boundary conditions of these effects are yet to be fully understood. This Special Issue offers a collection of research advances identifying mediating and moderating variables addressing when and why diversity impacts intergroup relational outcomes. By focusing on different levels of diversity (i.e., in the society and in groups), this research also sheds light on the effectiveness of ideologies and policies for managing diversity.