This article is based on a comparative study of three forms of evaluation used by government, the Audit Commission, consultants and the Social Services Inspectorate. The paper argues that these evaluative bodies strengthened the authority of government in the 1980s and supported the shift from a professional to a managerial culture in government. Their contribution to those changes can be linked to the kinds of knowledge they used and the positivist assumptions about knowledge that reasserted themselves in evaluation and in government in the 1980s. However, the research also provides further support for the critics of positivism. The distinction between fact and value and between technical expertise and political argument often broke down in evaluative practice.