This article reinterprets the constitutional theory of Emmanuel-Joseph Sieyès. Sieyès coined the notion of 'constituent power', which has since become one of the defining concepts of constitutional theory. However, as is argued in this article, Sieyès's conception of 'constituent power' has hitherto been largely misinterpreted due to the fact that Sieyès's theory has hitherto not been thoroughly situated within the entire corpus of Sieyès's writings as well as within the political conjuncture of 1788 and 1789; the short period during which Sieyès developed his constitutional theory. The concept of 'constituent power' is developed in pamphlets primarily concerned with the economic and political situation of France during the 'prerevolution', and these pamphlets argue for political strategy and action--something which existing research has failed to take sufficiently into account. This article therefore traces the development of Sieyès's concept of 'constituent power' in relation to his theory of political economy as well as his analysis of the political conjuncture -- the two elements that form the 'leitmotifs' of Sieyès's pamphlets. This reading of Sieyès's constitutional theory is preceded by a reconstruction of his theory of political economy, which forms the necessary basis for understanding his constitutional theory. The article thus gives a novel, profound and detailed understanding of Sieyès's concept of 'constituent power' and the role he thought it would play in revolutionizing France -- a conception that is still vital for understanding constituent power today.