This thesis is an analysis of the use and definition of the concepts health, well-being and welfare within the field called “the science of animal health and welfare”. The materials used are a literature survey of the field, qualitative interviews with Swedish veterinary surgeons and a study of the concepts in legislation concerning animals in England, Germany and Sweden. The main emphasis has been on theoretical definitions explicitly stated in the different texts or in the interviews. Two ways of distinguishing (...) between animals are used: according to species and according to the role that animals have for humans. As a result it becomes salient whether the definitions have limits with regard to species or role. In the thesis a great number of definitions of the three concepts are interpreted, compared and criticised. As a result a limited number of definitions have been categorised and collected into clusters which fulfil minimal requirements of consistency and practicability. The analysis supports the use of all three concepts – health, well-being and welfare – since they are all needed for making crucial distinctions in the science of animal health and welfare. (shrink)