This thesis sets out to address the issues raised by advances, in technology and to assess , the implications that they have for the use of non-human animals in scientific (medical) testing. I suggest that there are three factors which have led to a recent increase in the use of non-human animals in general and in genetically modified animals in particular. These are: Advances in the techniques of genetic modification; • Advances in our understanding of the role and function of genes, and the desire to improve this further; • The ever more competitive struggle to produce new drugs and treatments and to reward investors and shareholders.