This article focuses on the role of the Cortes in the course of two decisive moments of the Portuguese monarchy: the choice of a king on the one hand and the decision to accept a regency on the other. The Cortes of Coimbra (1385) are first analysed in the context of the political events that preceded them, in terms of the novelty of being summoned by the social forces of the kingdom, and in terms of their ideology and innovative measures. As a result, a king was chosen�King John I, the founder of the House of Avis�and reforms of a noticeable �constituent� tendency were proposed. Attention is then focused on the Cortes of Lisbon (1439), which were equally unusual in their summoning and performance. There, it was decided that the infant Don Pedro, brother to the late king Don Duarte, would be in charge of the regency while the heir to the throne, Don Afonso V, was not yet of age. This prevented the queen from occupying the role the monarch had intended for her. In both cases these Cortes are very special given that they legitimated revolutionary acts which had previously taken place. This behaviour reinforces the idea that by comparison with other kingdoms in the Iberian Peninsula, the Portuguese Cortes had a strong leadership role in moments of political crisis and in the assertion of the power and social formation of the monarchy.