What clinical oncologists learned about metastatic process, is that it is the main cause of cancer-related deaths. What scientists learned about metastatic disease, is that it is due to a highly selective process, which involves a minority of tumor cells that are able to survive within the bloodstream, and to initiate a new growth in distant sites. These cells "in transit" are known as circulating tumor cells (CTCs). Although their nature is not fully understood, what is widely accepted, is that they are drug resistant, and that their presence may represent the main reason for treatment failure. Despite this body of evidence, the pharmacological approach against cancer, with both chemotherapic and biological drugs, is still targeted on the primary tumor, raising the question as to whether we are missing the target. Targeting circulating tumor cells, may represent a new promising approach to indivisualize anticancer therapy.