Rationality is a central concept for epistemology, ethics, and the study of practical reason. But what sort of concept is it? It is argued here that—contrary to objections that have recently been raised—rationality is a normative concept. In general, normative concepts cannot be explained in terms of the concepts expressed by ‘reasons’ or ‘ought’. Instead, normative concepts are best understood in terms of values. Thus, for a mental state or a process of reasoning to be rational is for it to be in a certain way good. Specifically, rationality is a virtue, while irrationality is a vice. What rationality requires of you at a time is whatever is necessary for your thinking at that time to be as rational as possible; this makes ‘rationally required’ equivalent to a kind of ‘ought’. Moreover, rationality is an “internalist” normative concept: what it is rational for you to think at a time depends purely on what is in your mind at that time. Nonetheless, rationality has an external goal—namely, getting things right in your thinking, or thinking correctly. The connection between rationality and correctness is probabilistic: if your thinking is irrational, that is bad news about your thinking’s degree of correctness; and the more irrational your thinking is, the worse the news is about your thinking’s degree of correctness. This account of the concept of rationality indicates how we should set about giving a substantive theory of what it is for beliefs and choices to be rational.