Publisher Summary This chapter discusses the potentialities for present status of pharmacological research in genetically controlled mice. Laboratory animals are used increasingly all over the world. The greatest numbers, especially of mice, rats, and guinea pigs, are required for routine diagnosis of disease, bioassay of therapeutic substances, and screening of possible new remedies. These and others are important purposes for which laboratory animals are indispensable. Obviously, when speaking of laboratory animals it can only mean animals of the most suitable kind and satisfactory quality. In fact, the nature of many of the current purposes has created special requirements for animals conforming with definite specifications and often of a particular genetic constitution. Although utilization of all laboratory animals has grown to considerable dimensions there is a difference in relative proportions of different species used. Mice are the favorites nearly everywhere, followed by rats, guinea pigs, and rabbits.