The concepts of welfare, need, stress, health, pain, emotion and feeling are defined and the relationships amongst these are discussed. Welfare is a broad term, of which health and feelings are important parts. There has been rapid development in recent years in the scientific assessment of animal welfare. This information has been used in formulating laws about how animals should be housed, managed and treated. Where welfare is poor, the best overall assessment of welfare is a function of the severity of effect on the individual and the duration of that effect. Efforts should be made to evaluate how good welfare is as well as the extent of any poor welfare. © Wageningen Academic Publishers, The Netherlands 2008.