Metastases result from the implantation of tumor cells into various organs, distant from the primary tumor. Among primary tumor cells, a small number undergo genetic events determining metastatic function acquisition. The metastatic function comprises the lack of homotypic intercellular connections, due to proteolytic enzyme effects and/or lack of adhesion molecule synthesis, stimulation of peri- and intratumoral angiogenesis, motility adhesion molecule synthesis, stimulation of peri- and intratumoral angiogenesis, motility and invasiveness, adhesiveness to acellular substrates or heterotypic cells, resistance to immune defenses, alteration of growth factor receptivity, and even growth autonomy. Formation of metastatic foci needs the establishment of an ecosystem born from the contribution of both host and tumor and in which respective cells interact.