Knowledge use and production is complex and so also are attempts to judge its quality. Research synthesis is a set of formal processes to determine what is known from research in relation to different research questions and this process requires judgements of the quality and relevance of the research evidence considered. Such judgement can be according to generic standards or be specific to the review question. The judgements interact with other judgements in the review process such as inclusion criteria and search strategies and can be absolute or weighted judgements combined in a Weight of Evidence framework. Judgements also vary depending upon the type of review that can range from statistical meta analysis to meta ethnography. Empirical study of the ways that quality and relevance judgements are made can illuminate the nature of such decisions and their impact on epistemic and other domains of knowledge. Greater clarity about such ideological and theoretical differences can enable greater participative debates about such differences. (Contains 3 figures and 1 table.)