Close relationships among children and adolescents are ordinarily considered to encompass friendliness and fun. Recent studies, however, reveal that many friendships have dark sides consisting of competitiveness, hostility, and conflict. These attributes attenuate the contributions made by friendship experience to good developmental outcome (Berndt, Hawkins, and Jiao, 1999) and increase aggression and antisocial behavior (Dishion, Andrews, and Crosby, 1995).Social networks among children and adolescents also include relationships that are not based on social attraction at all, but rather are rooted in antipathy, animosity, and enmity. The term mutual antipathies describes this general category of relationships in which children and certain associates identify one another as persons whom they do not like. These mutual antipathies form a superordinate class of social relationships that encompasses "being enemies," as well as other relationships maintained on the basis of social aversion.Early studies of mutual antipathies and their developmental significance, including those described in this book, have raised a number of questions. We examine seven in this chapter:1. Conceptualization and methodology. Although some attention has been given to the need for making distinctions between relationship categories such as mutual antipathies, enemies, animosities, and aversions, considerable conceptual and methodological confusion still exists among