Chemoresistance of human malignant tumors causes the failure of anticancer agents for most carcinomas and for some hematological tumors. It is due to bad pharmacological conditions and also to resistance mechanisms at the cellular and at the molecular level. The identification and the comprehension of these resistance mechanisms as well as the possibility of circumventing them or preventing them might be responsible of a therapeutic progress in the next few years. However, at the present time, methodological process of identification and quantification of these mechanisms persist and the preliminary results of clinical trials with revertant drugs are still weak.