This chapter provides a foundation for those new to rivalry inquiry. First, it introduces seminal social psychology concepts, such as group identity, social identity theory, social categorization theory, and ingroup/outgroup formation. Next, the chapter explains three properties of rivalry and the 100-point single-item measure of rivalry intensity. Study 1 examines these in new leagues (MLB, MLS, NBA), finding robust support for rivalry as 1) non-exclusive (fans perceive multiple rivals), 2) continuous in scale (intensity varies among rivals), and 3) bidirectional (opposing fans rarely share equivalent perceptions of the rivalry). Study 2 explains 11 rivalry antecedents and investigates their manifestation within five sport leagues (MLB, MLS, NBA, NFL, NHL). These are, in descending order of influence: frequency of play, defining moments, recent parity, star factors, geography, relative dominance, historical parity, competition for personnel, cultural difference, unfairness, and cultural similarity. The authors close by noting limitations and future directions for rivalry research.