From a survey carried out in a Parisian museum, the article studies the professional practices of the commissaries of modern art exhibitions. The planning of these exhibitions, a process requiring an important co-operation between artists and commissaries, leads to taking a closer look at the modes of co-operation, rather than at the analysis of the conditions of the full powers allotted to the commissary. The training of the latter is achieved by direct contacts with plasticians and their work. Implying great mobility, it sharpens their abilities to make - and to justify - choices by cross-observing the work of the artist and his/her context. This empirism testifies of a conceptual method, partly common to the actors of modern art worlds. The exhibitions are thus an opportunity to “popularise” certain interpretative trends rather than others, and therefore, to boost those spheres. The commissary, for his part, with these common methods and through his choices, creates his own style, both readable and singular, by which he inscribes and displays his working ways.