Human attitudes to animals have changed as non-humans have become more widely incorporated in the category of moral agents who deserve some respect. Parallels between the functioning of humans and non-humans have been made for thousands of years but the idea that the animals that we keep can suffer has spread recently. An improved understanding of motivation, cognition and the complexity of social behaviour in animals has led in the last 30 years to the rapid development of animal welfare science. Early attempts to define welfare referred to individuals being in harmony with nature but the first usable definition incorporated feelings and health as part of attempts to cope with the environment. Others considered that welfare is only about feelings but it is argued that as feelings are mechanisms that have evolved they are a part of welfare rather than all of it. Most reviews of welfare now start with listing the needs of the animal, including needs to show certain behaviours. This approach has used sophisticated studies of what is important to animals and has replaced the earlier general guidelines described as freedoms. Many measures of welfare are now used and indicate how good or how poor the welfare is. Naturalness is not a part of the definition of welfare but explains why some needs exist. In recent years, welfare has become established as one of various criteria used to decide on whether a system is sustainable because members of the public will not accept systems that cause poor welfare. The study of welfare has become part of the scientific basis upon which important political decisions are made.