This chapter presents the book’s topic, i.e. law’s interaction with practical reasons, demarcates the scope of the book, and clarifies some key notions relevant to the inquiry. Specific issues commented on include, inter alia, the book’s working assumptions on the criteria of legal validity (Section 1.1.1); the relevance of arguments about authority (Section 1.1.2); and the type of reasons central to the inquiry (Section 1.2). Finally, the chapter sets out the two principal positions examined in Parts I and II: first, Raz’s position that when the law satisfies certain conditions that invest it with legitimate authority, it acquires pre-emptive force, namely it constitutes reasons for action that exclude and take the place of some other reasons; and, second, an antithetical position, the weighing model, which explains the relevant phenomenon in terms of reasons for action that compete with other reasons by means of their weight (Section 1.3).