The results of basic research of proliferation kinetics and pharmacokinetics as well as comparative clinical studies led to the development of therapeutic principles for malignant diseases. Thus polychemotherapy is to be preferred to monotherapy in most types of disease. To prevent primary resistance only those cytostatics should be combined which have been proved effective or could be expected to be effective individually. The relative cell cycle specificity of cytostatics should be taken into account by combination of drugs acting at different phases of the tumor cell cycle. The different growth behavior of tumor and normal tissues suggest the use of intermittent stoss therapy for the regeneration of normal tissue in the therapy-free interval.