The Harvard Law Review recently, for the first time, published Hart’s essay titled “Discretion”. It is a carefully arranged version of the lecture which he gave at Harvard in 1956. This essay fills significant gap in Hart's work concerning judicial reasoning. In my paper attention is devoted to his conception of judicial discretion, its two main types (express and tacit), and his understanding of interpretation and rationality related to Hartian discretion. According to Hart, discretion is a form of decision-making in hard cases, which is rational and to some extent constrained by law. However, because no combination of legal rules and principles, properly interpreted, will always give only one legally right answer, the judge in some cases must resort to non-legal reasons, i.e. exercise discretion. Hart’s insight that the law is not the sole ground for (judicial) decisions suggests that there is something “out there” (in our “practical universe”) that plays a role in the legal “earthly” world, and consequently, in the judicial world as well.