Popular sovereignty is regarded here as one of the fundamental concepts of modern law. Indeed, the current literature is dominated by a kind of sociological perspective, which reduces this concept either to a motivational problem, or to a procedure. In contrast to this view, popular sovereignty can be justified as a legal philosophical answer to the normative question of the legitimacy of law. The first part of this study will address the question of possible functions of the concept of popular sovereignty in law. In the second part, a historical/conceptual investigation of the concept of popular sovereignty will be developed on the basis of philosophical insights in the late Middle Ages (Thomas Aquinas, Marsilius of Padua and William of Ockham) and modern times (Jean Bodin, Thomas Hobbes, John Locke and Jean-Jacques Rousseau).