There has been a dramatic expansion and diversification of knowledge, expertise and expectation associated with farm animal welfare and we witness its increasing adoption within legislative and policy strategies. This article examines how the understanding of what constitutes farm animal welfare and how it should be enhanced, has shifted since it first entered the modern political arena in the sixties. It explores farm animal welfare as a critical and shifting area of imbrication of ‘science’ and ‘society’ and charts this process in particularly the UK and The Netherlands. We demonstrate how public and scientific debates about what constitutes animal welfare, what counts as evidence of animal suffering and how welfare enhancement should be encouraged, has unveiled the tensions underlying its construction as a policy problem between social and animal science, productivity and integrity, as well as production and consumption and has turned animal welfare into a matter of societal choice. The article unravels the social construction of animal welfare science and reveals its limited capability of delivering ‘facts’ or ‘evidence’ in any truly objective manner.