It is hard to imagine a good life without friendship. But what precisely makes friendship so valuable? And what is friendship at all? What unites friends and distinguishes them from others? Is the preference we give to friends rationally and morally justifiable? This collection of new essays on the philosophy of friendship considers such questions. In particular, it examines and offers new interpretations of the answers given by famous classic philosophers such as Plato, Aristotle, and Kant and provides fresh answers by leading contemporary philosophers. It is systematically organized around five topics: the nature of friendship, the unity of friendship, friendship and reason, friendship and morality, and the place of friendship in a good life. In this way it discusses the main problems of the philosophy of friendship from both historical and contemporary perspectives.