What kind of instrument is PISA? Does it resemble a mechanism of the Evaluative State, that is, is it an instrument useful in decision -making? Alternatively, does it perhaps constitute an instrument required for surveillance in the way Rosanvallon (2006) uses the term? Perhaps should we view it as an instrument facilitating soft regulation? We will compare each of these propositions with our empirical observations. Unlike the official position, which views PISA as an instrument supporting decision -making, our findings indicate that the instrument — at least in the context of French -speaking Belgium — tends instead to function as a monitoring instrument that increases the pressure of the quasi-market on (public and private) providers of education. According to this analysis, the State should not be viewed solely as an evaluative State; it must also be evaluated, ranked and compared to other purveyors of educational services, both Belgian and foreign. The analysis prompts us to propose a new concept, that of “third-party evaluator†(Rosanvallon 2006), which could enrich and refine analysis of the role of knowledge in public policy.