An animal model is a living organism in which normal biological processes can be studied, or in which a spontaneous or induced pathological process can be investigated. To be effective, the process being modeled should closely resemble the analogous process in human beings or some other species in one or more ways (cf. Held, 1983). Some important criteria of animal models are: relevance to the problem being studied; the accuracy with which the model reflects all or some important aspects of the problem; the model's predictability; and the model's availability to researchers. In addition, general species characteristics such as life history parameters, behavior and diet can be as important as physiological parameters in species choice. Some ways in which these factors may affect a species' suitability as a model are discussed below.