Interaction of Fas-ligand (Fas-L) with the extracytoplasmic domain of the Fas receptor can induce Fas trimerization and activation of the apoptotic cell death process. Several molecular pathways that lead to apoptosis and some of their regulatory mechanisms have been identified. Fas-related membrane receptors that contain a death domain in their intracytoplasmic domain have been identified. They constitute a death receptor family (DR1 to DR5) whose first member is the TNFR1 receptor for TNF alpha. The Fas/Fas-L system plays a role in the cytotoxic activity of immune cells and the regulation of immune response amplitude. This system could be involved in the immune response to tumor cells and the cytotoxic activity of drugs and radiations. The expression of Fas-L on the plasma membrane of numerous tumor cells allow them, in vitro, to kill Fas-expressing immune cells. This observations has suggested that tumor cells used Fas-L to induce a specific immune tolerance. However, in vivo, Fas-L expression rather induces tumor cell rejection. The quantity of Fas-L expressed on tumor cells could determine whether tumor cells are tolerated or rejected. Cytotoxic drugs and radiations modulate Fas and Fas-L expression on tumor cells. The role of Fas/Fas-L interactions in the cytotoxicity of these agents remains poorly defined. It has been clearly shown, however, that low doses of cytotoxic drugs increase Fas expression on tumor cells, thereby improving their elimination by immune cells. Drug-induced modulation of Fas expression could provide new therapeutic strategies combining chemotherapy with immunotherapy.