Metastatic progression consists in multiple, progressive, well structured and organized steps, called the metastatic cascade. These steps are becoming clearer, for example, angiogenesis which is absolutely necessary for tumor growth but also for metastatic colonization in the new organ. There are other concepts such as epithelial mesenchymal transition, the premetastatic niche or the metastatic signature theory. They all participate in an irreversible process which escapes from the host control. Tumor progression is highly dependent on genes that mediate the process but also on important extrinsic phenomena such as the tumor microenvironment (basement membranes, the extracellular matrix). The capacity to colonize one organ and not another could be explained by the "seed and soil" theory which postulates that tumor cells (the seed) will only grow in an.