Knowledge and expertise are not only the property and intellectual capital of individuals, but also an important asset for the whole organization. Knowledge management is a challenging task; in order to be successful, one should not only understand the different practices within the company, but also know what the content of the work is like at different organizational levels and functions. The SafeExpertNet project focused on studying the work of nuclear experts, and identifying how the organization can support the preservation and development of the expertise. One of the objectives of this project was to identify such leadership (and HR practices) that support the preservation and increase of expertise in nuclear industry. Therefore, the focus of this paper is on the role of supervisor in knowledge management and expertise development. In this four-year project different datasets were collected: thematic interviews concerning the nature of the expertise, its development, and the organizational support for developing expertise (n = 29, in 2007) and how the safety critical aspect of their work should be taken into account in management, supervisor and team working (n = 12, in 2009) have been conducted. Surveys on the organizational practices at work in the improvement of expertise (n = 170 in 2008, n = 279 in 2010) and on initiation practices (n = 32 in 2009) have also been carried out. All in all, the findings of the project confirmed that the role of the supervisors is demanding and significant in the knowledge management and development of expertise. The challenge of the supervisors is not only to support the development of expertise of the others, but also make sure that they can also develop their own expertise and share it with the new experts, since often the supervisors in nuclear industry organizations in Finland are also the main experts in their area.