In the midst of the ultra modernity of stock exchanges and financial markets, certain practitioners are increasingly using forecasting models of changes in rates whose scientific rationality is particularly contested. We refer to ‘technical analysis’, or, in the jargon of finance, of ‘chartism’. The adherents of this practice affirm that their models offer the possibility of detecting and of reading, by means of stereotyped graphical configurations, rising or falling market trends. Now, for a large number of people who hold scientific economic and financial theories to describe the ‘nature’ of changes in rates as so many random and unpredictable phenomena, these models arise above all from the sphere of ‘superstition’ and ‘beliefs’, in the worst sense of the term. To listen to them, one would think there was hardly any difference between sorcery, prophecy, and chartism, since in short: “it works because people believe in it”. These are remarks that can only arouse the curiosity of the anthropologist, because they actually serve merely to recall epistemologica! controversies that depend on the rationality and the irrationality of observed practices and debates in various cultures. At the heart of these contradictory debates, it is precisely prophecy as a cultural mode of managing unforeseen events that has been the subject of multiple anthropological, historical, and epistemological studies which can, in a comparative perspective, clarify the structure and functioning of forecasting tools employed by the chartists in the context of financial speculation.