What is the economic role of diplomas on the labour market? In terms of filter theory, they serve to signal an individual's productivity, and thus contribute to making the higly competitive market fair and open; in terms of segmentationn theory, they serve as a passport on the more highly administrated markets. These two hypotheses, which lead to radically different conclusions as to the need for education, both involve the idea that the certifying institutions are external entities which affect the way the market works. Historically, certification can be said to be a socially constructed form of extra-market rationing of workers' access to their means of subsistence. Assuming diplomas to be one of the ways in which the relationships between the use and perpetuation of the workforce are regulated yields a typology of the logic between certification and workforce management and suggests some hypotheses to account for the factors responsible for the changing role of diplomas, both as an entry-card to the labour market and as an instrument for steering systems of education.