Despite significant improvement in surgical techniques and chemotherapies, none of the current medical technologies "cure" metastatic disease, and the patients who have acquired metastatic cancer inevitably die from disseminated disease. Thus, there is a need for developing novel therapeutic approaches which can directly target metastatic tumor cells. However, advances in understanding the molecular mechanism of tumor metastases have lagged behind other developments in the cancer field. Tumor metastasis involves complex array of steps with each step requiring a coordination of the actions of many positive and negative factors. A number of tumor metastasis suppressors have been identified which suppress the formation of tumor metastasis without affecting the growth rate of the primary tumor. Such discoveries offer new approaches for curtailing tumor metastasis. This review summarizes our current understanding on these genes and their potential role in the progression of tumor metastases.