The National Institute for Health and Care Excellence (NICE) is the UK's primary health care priority-setter, responsible for advising the National Health Service on its adoption of health technologies. The normative basis for NICE's advice has long been the subject of public and academic interest, but the existing literature does not include any comprehensive summary of the factors observed to have substantively shaped NICE's recommendations. The current review addresses this gap by bringing together 29 studies that have explored NICE decision-making from different disciplinary perspectives, using a range of quantitative and qualitative methods. It finds that although cost-effectiveness has historically played a central role in NICE decision-making, 10 other factors (uncertainty, budget impact, clinical need, innovation, rarity, age, cause of disease, wider societal impacts, stakeholder influence and process factors) are also demonstrably influential and interact with one another in ways that are not well understood. The review also highlights an over-representation in the literature of appraisals conducted prior to 2009, according to methods that have since been superseded. It suggests that this may present a misleading view of the importance of allocative efficiency to NICE's current approach and illustrates the need for further up-to-date research into the normative grounds for NICE's decisions.