Societies have long turned to experts for advice on controversial matters, but in the past, the arrangements to solicit expert advice were largely ad hoc. In recent years we have witnessed the development of an institutionalized system in which scientists offer knowledge in exchange for influence on the policy process, creating, in effect, a permanent assessment economy. We examine this process of expert assessment through detailed analyses of three groups of large, formal scientific assessments: the U.S. National Acid Precipitation Assessment Program, international assessments of ozone depletion, and assessments examining the potential disintegration of the West Antarctic Ice Sheet. We show that assessments not only summarize existing knowledge, but also can create new knowledge and set research agendas. Assessments can also impede the development of knowledge, particularly if scientists focus unduly on uncertainty or on achieving consensus. The desire to achieve consensus can also weaken assessment outcomes by leading scientists to converge on least common denominator results. Assessments often try to stay on the science side of a poorly defined and intermittently enforced boundary between science and policy because of a concern with objectivity and efficacy. Assessments often try to neutralize bias by being inclusive in terms of nationality, gender, and prior intellectual commitments—adopting what we call a “balance of bias” strategy. We conclude that the assessment process is one of expert discernment, but nevertheless surprisingly sensitive to the institutional arrangements that establish it.