Standard methods of impact evaluation often leave significant gaps between what we know about development effectiveness and what we want to know—gaps that stem from distortions in the market for knowledge. The author discusses how evaluations might better address these knowledge gaps and so be more relevant to the needs of practitioners. It is argued that more attention needs to be given to identifying policy-relevant questions (including the case for intervention), that a broader approach should be taken to the problems of internal validity (including heterogeneity and spillover effects), and that the problems of external validity (including scaling up) merit more attention by researchers.