This book addresses the classical but nonetheless still controversial question of what the role is of judicial discretion in adjudication. Its main purpose is to discuss the philosophical and epistemic foundations of two different legal theories: Hartian positivism and Dworkin's interpretivism. Bearing in mind the debate between realism and antirealism, the author shows how Hartian positivism faces serious difficulties to avoid global scepticism, and the problem of rule-following by simply connecting legal determinacy with the existence of settled conventions. In contrast, it is argued that Dworkin's interpretive theory can overcome scepticism by connecting legal determinacy with the idea of the best interpretation, and by rejecting externalism. With a view to justifying this claim, the author presents a reconstruction of Dworkin's philosophical position, which developed along the lines of Putnam's internal realism and Rawls' reflective equilibrium. This book will be of interest to legal theorists, lawyers, judges, and philosophers.