Since the beginning of scientific medical research, animal experiments have played a decisive role in detecting and treating diseases. In the past there were numerous spectacular successes in medical research which became possible only by using an animal experiment. The infection of guinea pigs with tubercle bacilli and subsequent treatment of the disease with “tuberculin” formed the basis for successful detection and treatment of tuberculosis. The development of antidiphtheritic serum by Emil von Behring was possible only by the immunization of wethers. Mention should also be made of Landsteiner’s detection of the blood groups and the importance of rhesus monkeys in the blood typing of subgroups. In recent times, mention must be made of experiments in rats. These were designed for the induction of a disease pattern similar to Morbus Parkinson, by means of reserpine, and for the extrapolation of the experimental results to man, since these experiments initiated the detection and investigation of dopamine. Goats played a decisive role in the immune diagnostics of cardiac infarcts. We have mentioned these five examples, which may be supplemented at will, because they show that special problems demand special animal species for their solution.