The use of animals in experimental research parallels the development of medicine, which had its roots in ancient Greece. The increasing demand for high-standard animal models, together with a critical view of the way animals are used, has led to the development of a multidisciplinary branch of science we now know as 'laboratory animal science'. The guiding principles are replacement, reduction and refinement (the Three Rs), first proposed by Russell and Burch in 1959. When animals are used, the people involved have an obligation to safeguard their welfare and minimise discomfort; this will also generally be beneficial for both the animal and the experimental outcome. The ability of an animal to cope with the environment and exert control over its life seems to be crucial for animal welfare. In this paper, attention is paid to the assessment of welfare, environmental factors affecting welfare, legislative requirements and future trends such as the production and use of genetically modified animals.