The concept of a reason is now central to many areas of contemporary philosophy. Key theses in ethics, epistemology, political philosophy, philosophy of action, and the philosophy of the emotions, among others, have come to be framed in terms of reasons. And yet, despite their centrality, theorists seem to take inconsistent things for granted about how reasons work, what kinds of things can be reasons, what reasons favor, and more. Somehow reasons have come to be both indispensable and impenetrable. The Fundamentals of Reasons offers a comprehensive introduction to the philosophy of reasons. Focusing on the twin roles of reasons in explanation and deliberation, the book not only emphasizes what has made reasons central across philosophy but it also explores why philosophers have such incompatible pictures about what reasons are and how they work. Working from the inside out, Howard and Schroeder identify contentious assumptions about not only the internal structure of reasons but also their relationship to other important concepts, and then show how these contentious assumptions shape the many downstream applications of reasons in ethics, epistemology, political philosophy, and beyond. This mildly opinionated exploration of key questions about the significance and nature of reasons helps the reader to navigate this important part of the philosophical landscape and to get clearer about why reasons seem important and what their import, ultimately, is.