Abstract : In France, as in many other OECD countries, a sharp increase in the number of Ph.D. graduates in S&E fields from the mid-1980s to the mid-1990s was followed by a decrease in the enrolments in Ph.D. studies. Important transformations in the innovation processes were also observed on the same period. Our hypothesis is that we assist to the emergence of an intermediate scientific labour market between the traditional academic sector and the private research sector. A detailed analysis of the production processes of scientists and of the scientific careers of young scientists is first carried out in the perspective of labour economics. Then, in the perspective of the economics of innovation and technical change, the characteristics of young researchers' jobs and R&D activities are studied. We try to isolate three scientific communities: academics and researchers in the public sector of research, researchers in the private sector and researchers in the intermediate research sector.