In debates about the welfare of animals, different people have tended to emphasize different concerns. Some emphasize the basic health and functioning of animals, especially freedom from disease and injury. Others emphasize the "affective states" of animals – states like pain, distress and pleasure that are experienced as positive or negative. Others emphasize the ability of animals to live reasonably natural lives by carrying out natural behaviour and having natural elements in their environment. These concerns constitute different criteria that people use to assess animal welfare. The criteria overlap substantially but are sufficiently independent that the single-minded pursuit of any one criterion may lead to poor welfare as judged by the others. The different criteria reflect different sets of values that have been in conflict since the early debates about human welfare during the Industrial Revolution, with one side valuing a simple, natural life while the other values progress, productivity, and a life improved by science and technology. Scientific research on animal welfare has been based on the various criteria of welfare. Such research has helped to identify and solve animal welfare problems through improved housing and management of animals. However, the research has not resolved the differences attributable to the different criteria of animal welfare. Rather, the different criteria have provided the rationale for diverse approaches to animal welfare research. Thus, our understanding of animal welfare is both values-based and science-based. In this respect, animal welfare is like many other topics of "mandated" science such as food safety and environmental sustainability where the tools of science are used within a framework of values.