The legitimacy of policies does not rest only on procedures that are formally correct, but on the quality of the information and real knowledge that support the planning of an intervention and guarantees that it tackles the «problem». But there was a crisis in the authority of expert knowledge in the 1970s. The article examines the reasons for this, the range of solutions suggested by the movement for participatory analysis and evaluation, and the criticisms raised by the results of these practices; in conclusion it identifies an alternative that at the moment has been left open. only subscribers can see the full article