The argument of this chapter is that brotherhood can be conceptualized as a partially organized relationship, based on membership and rules. To illustrate the conceptualization, the chapter draws examples from three different arenas where there is a strong rhetorical emphasis on brotherhood, or fraternity: the military, motorcycle clubs, and monasteries. Membership determines who is a brother or not and while the brotherly relationship sometimes extends beyond the cessation of membership in a formal organization, it presupposes membership at some point. Rules clarify important components of brotherhood including homogeneous relations among all brothers (or sisters). This makes a crucial difference relative to friendship, which is a type of relationship that can even be a threat for brotherhood. In areas where collectivist ideology, homogeneity of relationships, and requests on loyalty are especially forceful, personal or friendly relations between individual members cannot compensate for failure as a “brother.” Brotherhood justifies sacrifice of individual needs to collective demands, and this may include the sacrifice of a personal relation.