This article contributes to unraveling the 'paradox of scientific authority', that is, the fact that despite the loss of authority of scientific expertise, policymakers still resort to expert advice. Re-examining the role ascribed to expert assessment in the policy-making process in controversial contexts in particular, the article succeeds in demonstrating that one of the crucial roles of expert evaluation is to establish a more compelling definition of the problem to be dealt with by policymakers. Taking the scientific controversy surrounding the proliferation of green algal bloom on Brittany beaches (France) as a case in point, I show that expert assessment conceived as a framing exercise is, however, a two-way process: it is as much about framing for the sake of settling an expert dispute with sound scientific categories than about solving public problems in a sufficiently consensual way, taking into account the distribution of power more generally in society.