Abstract : As testify used vocabulary, contemporary training for a future librarian introduce progressively the question of the work by the way of corporate culture rather than the concrete work, the activity. I suggest to substitute this external approach, that meet necessary resistance in the job, for an intern approach by analyzing in first the notion of activity, then the one of documentary activity, concerning a public or a private service. Because of the originality of this approach, the analysis of the activity require first an inquiry on our cultural heritage, the history of ideas. It shows that the activity respond imperfectly at a logical or conceptual questioning, because the activity is anterior to all conceptualization. However, activity study can use ordinary language resources to give rise to ergology, understand as the joint between knowledge provided by experience, disciplinary knowledge, and debates about values around the activity. Thus, in any qualities about research on fundamental concepts of the documentary activity, those researches invariably improve to divide and reduct the wealth of abilities and how-know invested, however chosen the approach may be : cultural, representational, deductive or inductive. And to give a concrete form at our ergologic approach of the librarian job, I describe few examples of application in a situation of work. So, the documentary ergology invite to renew studies on the work in library while keeping up knowledge come from concrete practice.