Between ideology and utopia, one of the functions of imagination is to give an almost tangible appearance to concepts, notions and ideals which are thought, a priori, not to have any. Technical imagination thus helps to create imaginal representations of efficiency. This article considers two key stages in the history of these representations. The first period, corresponding roughly to Michel Foucault's 'classical age'?, is marked by the importance of what could be seen as architectonic representations of efficiency. Transformations of the ideal of efficiency during the Enlightenment are then studied. The last section, more prospective than historical, looks at the contemporary evolution of the search for efficiency and the paradigmatic role therein of information technologies.