Sometimes our intentions and beliefs exhibit a structure that proves us to be irrational. Is there anything wrong with that? Should we be rational rather than irrational? This is the question that this book seeks to answer. Intuitively, the answer to this question is ‘yes’. Calling someone irrational amounts to a form of criticism. By doing so, we seem to imply that the person in question has made some kind of mistake, that her mental attitudes are in need of revision. Ordinary attributions of irrationality thus seem to presuppose that rationality is normative. This understanding is also implicit in many traditional approaches to rationality. In recent years, however, the normativity of rationality has come under attack. Many philosophers today accept the sceptical view that there may be no reason to be rational. This book defends the normativity of rationality by presenting a new solution to the problems that arise from the common assumption that we ought to be rational. The argument touches upon many other topics in the theory of normativity, such as the form and the content of rational standards or requirements, the preconditions of criticism, and the function of reasons in deliberation and advice. Over and above an extensive and careful assessment of the problems discussed in the literature, the book provides a detailed defence of a reason-response conception of rationality, a novel, evidence-relative account of reasons, and an explanation of structural irrationality in terms of these accounts.