In response to an increased public awareness regarding how livestock are reared, animal welfare scientists have attempted to develop new methods of welfare assessment at the farm level. Furthermore, in recent years they have increasingly moved away from the conventional approach of evaluating the provision of resources necessary to ensure good welfare, and have instead focused on the use of animal-based measures of welfare. In contrast, it is believed that farmers use mostly resource-based and management-based measures (eg the provision of food, water and housing) when assessing the welfare of their animals. They also seem to be driven more by economic and financial concerns than by the welfare of the animals per se, when it comes to the provision of animal welfare. Different approaches to the definition and assessment of farm animal welfare were explored in work carried out at Newcastle University as part of the Welfare Quality® project by both social and welfare scientists. Social scientists explored farmers' perceptions and understanding of animal welfare, whilst welfare scientists developed animal-based measures of welfare for use in a prototype on-farm welfare monitoring system. Based on two separate surveys, this paper focuses on UK farmers' perception and understanding of animal welfare and their criteria of assessment in contrast with those employed by welfare scientists, using a specific case study of pigs. Results show that, despite scientists being unaware of the findings from the farmer survey, they produced a set of measures to assess welfare which were very similar to those used by farmers. However, 'instinctive' terms used by farmers to describe (positive or negative) animal behaviour did not bear any relation to more objective welfare measures. Compared with conventional monitoring systems which focus more on the provision of resources to promote good welfare than on the animal itself, the prototype monitoring system may be more acceptable to farmers given that it uses similar animal-based measures to assess welfare to those they use themselves, and furthermore, the focus is on the animal.