For much of the past two millennia philosophers have embraced a priori knowledge and have thought that the a priori plays an important role in philosophy itself. Philosophers from Plato to Descartes, Kant to Kripke, all endorse the a priori and engage in a priori reasoning in their philosophical discussions. Recent work in epistemology and experimental philosophy, however, has raised questions about both the existence of a priori knowledge and the centrality of the a priori for philosophy. This collection of essays aims to advance the discussion of the a priori and its role in philosophy by addressing four issues. The first is whether intuitions provide evidence for philosophical propositions, whether that evidence is a priori, and whether the results of experimental philosophy affect the evidential and a priori status of intuitions. The second is whether there are explanations of the a priori and what range of propositions can be justified and known a priori. The third is whether a priori justified beliefs are needed in order to avoid some skeptical worries. The fourth is whether certain recent challenges to the existence or significance of the a priori are successful. The contributors include a mix of young and established philosophers, including some of the most prominent voices in philosophy today.