Prioritarianism is a principle of distributive justice. Roughly, it states that we should give priority to the worse off in the distribution of advantages. This principle has received a great deal of attention in political theory since Derek Parfit first introduced the distinction between egalitarianism and prioritarianism in his Lindley Lecture, published in 1991. In the present article, prioritarianism is defined in terms of a number of structural features of the principle. These structural features are also used to distinguish between this principle and other distributive principles such as utilitarianism, egalitarianism, and leximin. Prioritarianism is mostly discussed as an axiological principle that orders outcomes with respect to their (moral) value, but it is also clarified how it can be incorporated in a criterion of right actions, choices, or policies. Furthermore, different aspects of the principle that need to be further specified to arrive at a full-fledged distributive theory are discussed, including the weights that give priority to the worse off, currency (what kind of advantages should be distributed), temporal unit (the temporal span in which one has to be worse off in order to be entitled to priority), scope (whether the principle applies globally or only domestically, and whether, for example, future generations and non-human animals are covered by the principle), and risk. For each aspect, different possible views are distinguished and discussed. Finally, it is discussed how prioritarianism may be justified, for example, by outlining and discussing the argument that, unlike certain other distribution-sensitive principles such as egalitarianism, prioritarianism is not vulnerable to the so-called “leveling down objection.”