In the present article, the steps involved in the process of tumor metastasis are discussed. Several events are required for malignant cells to leave the primary tumor and proliferate at a distant site: vessel formation (angiogenesis), cell attachment, invasion (matrix degradation, cell motility), and cell proliferation. Molecular mechanisms underlying each of these steps are described. Based on blocking these processes, new anti-metastasis therapies are being developed. Cancer 1997; 80:1529-37. © 1997 American Cancer Society.