This article examines the fortune, paradoxical in many respects, of the Lettre à d’Alembert sur les spectacles in French theatre theory and practice during the mid-twentieth century. While Rousseau seems to suggest that the theatre is morally and politically noxious, his ideas, similar to Diderot’s, have left their mark on theatre reformers, like Louis-Sebastien Mercier (eighteenth century) and Romain Rolland (nineteenth century). The latter assured the posterity of these ideas in his own theater through his own experience of theatre, that is, of Maurice Pottecher in Bussang, of Gemier and, during the Libération, of Jean Vilar and the Festival d’Avignon. After the war, Rousseau’s memories permeate Jean Vilar’s theories and texts from the first issues of the journal Théâtre populaire, particularly in articles by Roland Barthes and Jean Duvignaud. The critique of the social experience determined by Italian-style theatre and its replacement by new experiences in large and open venues (as in Avignon) are based upon Rousseau’s thought. The theorist’s influence is also palpable when one considers how the fête was conceptualized from 1954 until the early 1970s.