The critical appraisal and quality assessment of primary research are key stages in systematic review and evidence synthesis. These processes are driven by the need to determine how far the primary research evidence, singly and collectively, should inform findings and, potentially, practice recommendations. Quality assessment of primary qualitative research remains a contested area. This article reviews recent developments in the field charting a perceptible shift from "whether" such quality assessment should be conducted to "how" it might be performed. It discusses the criteria that are used in the assessment of quality and how the findings of the process are used in synthesis. It argues that recent research indicates that sensitivity analysis offers one potentially useful means for advancing this controversial issue.