This chapter discusses animal models of toxoplasmosis with special regard to pharmacological applications, and thereby tries to update existing reviews. All mammals can be infected with toxoplasmosis. However, different animal species differ markedly in their resistance to Toxoplasma infection. In addition, the outcome of infection is dependent not only on the animal species but also on the animal strain. The genetic background seems to be of importance since, after infection with Toxoplasma, striking differences in susceptibility of various strains of inbred and outbred mice are observed. The situation becomes even more complex because these differences are not uniform with respect to the strains but are also a function of the mode of inoculation. In addition to host factors, the outcome of a challenge with Toxoplasma is largely influenced by the nature of the infectious agent itself, and one of the most common characteristics of many Toxoplasma strains is the variation in virulence. Depending on the time before animals succumb to infection or the percentage of animals that do succumb, highly virulent, moderately virulent, and less virulent strains have been characterized.