The idea of a reason for acting is ubiquitous in philosophy, social science, the law, and in our everyday lives. Just about everyone understands the idea of a reason for acting, at least on an intuitive level. When someone does something intentionally, we often want to know why they did it: what their reason for acting was, or, perhaps equivalently, their goal, intention, purpose, or motive for acting. We also ask whether their reason was a good one and so whether their reason genuinely justified their action. Reasons, thus, play both justificatory and explanatory roles. Whether someone acted for a good reason may be a matter of significant consequence, as when our reasons for acting bear on our moral character or our legal liability.Against contemporary orthodoxy, Reasons Last argues that there is a deep unity between these explanatory and justificatory reasons, typically called motivating and normative reasons. Samuel Asarnow asks what happens to our theories of reasons if we take that unity seriously, instead of developing theories of motivating and normative reasons separately. He then develops, in detail, a unified and plausible account of these types of reasons for action.Asarnow's argument supports two novel theses: The first is the eponymous Reasons Last principle. Against the recently fashionable Reasons First thesis, which hypothesizes that reasons come first in normative theory, Asarnow argues that the idea of a reason for action comes last--or, at any rate, pretty far along--in the order of analysis. The second thesis is the Reasoning View about reasons for action, which posits that reasons for action are premises in possible pieces of reasoning about what to do. The Reasoning View contrasts with Reasons First views, but also with "explanationist" views according to which normative reasons are elements in explanations of normative or evaluative facts. Asarnow argues that the Reasoning View is the most promising way to develop the Reasons Last approach, and that it allows us to find unity between instrumental rationality, broadly understood, and reasons-responsiveness. Finding this unity will help us better understand the nature of reasons for action, questions of whether morality is universal, and the nature of intentional action.