Friendship is a relationship between two people who hold mutual affection for each other. Friendships and acquaintanceship are thought of as spanning across the same continuum. The study of friendship is included in the fields of sociology, social psychology, anthropology, philosophy, and zoology. Various academic theories of friendship have been proposed, including social exchange theory, equity theory, relational dialectics, and attachment styles.

Friendship, as understood here, is a distinctively personal relationship that is grounded in a concern on the part of each friend for the welfare of the other, for the other's sake, and that involves some degree of intimacy. As such, friendship is undoubtedly central to our lives, in part because the special concern we have for our friends must have a place within a broader set of concerns, including moral concerns, and in part because our friends can help shape who we are as persons. Given this centrality, important questions arise concerning the justification of friendship and, in this context, whether it is permissible to “trade up” when someone new comes along, as well as concerning the possibility of reconciling the demands of friendship with the demands of morality in cases in which the two seem to conflict.

Friendship essentially involves a distinctive kind of concern for your friend, a concern which might reasonably be understood as a kind of love. Philosophers from the ancient Greeks on have traditionally distinguished three notions that can properly be called love: agape, eros, and philia. Agape is a kind of love that does not respond to the antecedent value of its object but instead is thought to create value in the beloved; it has come through the Christian tradition to mean the sort of love God has for us persons as well as, by extension, our love for God and our love for humankind in general. By contrast, eros and philia are generally understood to be responsive to the merits of their objects—to the beloved's properties, especially his goodness or beauty. The difference is that eros is a kind of passionate desire for an object, typically sexual in nature, whereas ‘philia’ originally meant a kind of affectionate regard or friendly feeling towards not just one's friends but also possibly towards family members, business partners, and one's country at large (Liddell et al., 1940; Cooper, 1977a). Given this classification of kinds of love, philia seems to be that which is most clearly relevant to friendship (though just what philia amounts to needs to be clarified in more detail).

For this reason, love and friendship often get lumped together as a single topic; nonetheless, there are significant differences between them. As understood here, love is an evaluative attitude directed at particular persons as such, an attitude which we might take towards someone whether or not that love is reciprocated and whether or not we have an established relationship with her. Friendship, by contrast, is essentially a kind of relationship grounded in a particular kind of special concern each has for the other as the person she is; and whereas we must make conceptual room for the idea of unrequited love, unrequited friendship is senseless. Consequently, accounts of friendship tend to understand it not merely as a case of reciprocal love of some form (together with mutual acknowledgment of this love), but as essentially involving significant interactions between the friends—as being in this sense a certain kind of relationship.