Animal models are frequently used in medico-biological studies. The why and wherefore of animal models, the question of the requirements which have to be satisfied by an animal model, the pros and cons of animal models, the question of alternatives, the types of animal models available, the choice that has to be made and the manner in which an animal model is used, are discussed in succession. In addition, the animal models and techniques in use in the Central Animal Laboratory of the University of Nijmegen are reviewed.