Animal models are widely used in preclinical studies in order to explain the mechanisms of action of chemotherapy, radiotherapy and concomitant chemoradiation, to analyze pathophysiology of tumors and to evaluate the treatment of choice for malignant tumors. The choice of murine tumor or human tumor xenograft system is still debated. Xenografted human tumors have two main advantages: their human origin and wanted pathological type, which is necessary for future clinical studies. There are a lot of disadvantages of xenografted tumors: the stroma and vascular network of transplanted tumors have murine origin, the graft is mostly ectopic, the volume of transplanted tumors at the time of chemoradiotherapy is much smaller than that of the tumor in man; due to residual immunity it is difficult to determine response to cytotoxic treatment. It is still impossible to extrapolate the results obtained in a tumor model in animal to man. These investigations are usefull for interpretation of clinical results and for proposing the less empirical method of chemoradiation for phase II clinical trials.