The paper reviews theoretical mainstreams of Business and Industrial Organization, from Property Right paradigm to New Institutional Economics and to Organization Economics. These approaches allow to revisit the classic issue of the special features of public firm and consequently the problem of the mechanisms of government intervention on public firms. Whether or not the public ownership would be legitimate, this is the underlying and key question. This approach is then applied to the analysis of French public sector evolution, characterized both by privatization process and current achievement of European Market. It demonstrates the efficiency of mixing theoritical and historical approaches.Colette Voisin Nature et légitimité des entrepri...