[en] Full text: In a context of growth and important renewal of the teams (due to the workforce ageing), the Nuclear entity of Tractebel Engineering, which is strongly aware of the importance o maintaining critical knowledge, has set up a program of knowledge management explicitly aimed at training those new forces, improving the collaboration between colleagues and capitalizing on knowledge. This paper addresses in a concrete way one of the actions carried out: the transfer of expertise. It presents the developed methodology, the results of the projects initiated since 2004 as well as the lessons learned. This expertise transfer intends to respond to the problems of the departure of certain personnel who possess a critical expertise for the realization of the activities and at preventing the losses of competences worked out with the passing of years. Even if there always has been some amount of informal transfer of expertise between seniors and juniors, it has appeared essential, since several years, to structure a process of knowledge transfer that would make sure that critical and strategic knowledge will be well preserved and transmitted so that know-how remains and renews itself. In addition this process contributes to make the teams aware of the importance for the company of the knowledge transfer and capitalization even if not directly related to workforce replacement. (author)