During the natural history of a tumor, cancer cells become more and more aggressive and their increasing malignancy leads usually to the patient's death. The expression of malignant properties by tumor cells is manifested by the occurrence of metastases and is the result of an overexpression of molecules that are normally or barely non expressed by the normal cell progenitors. These molecules can be involved in cell attachment (receptor to the extracellular matrix), in proteolysis (collagenases), in angiogenesis (b FGF), in adhesion to endothelial cells, in resistance to the immune system. The genetic instability of tumor cells favors the amplification, mutation and gene translocation events, resulting in the activation of some genes or/and oncogenes which might direct the expression of the malignant properties. Finally, metastatic cells have been shown to have a growth advantage over non metastatic cells, so that metastatic cell population becomes ultimately numerously dominant in the primary tumor. The current knowledge about the malignant cell properties allow us to begin to understand how a cancer cell becomes metastatic and how the metastatic dissemination is usually an ineluctable process.