Tumors usually reach secondary sites via blood vessels or lymphatic vessels. Two processes dependent upon cell migration speed metastasis by reducing the distance between the primary tumor and these vessels. The first process is invasion, in which cancer cells migrate toward the capillaries. The second is angiogenesis, blood vessel growth into the primary tumor, which has been proved to promote blood-borne tumor spread by reducing the invasive distance, and may also aid lymphatic metastasis. Angiogenesis is dependent on endothelial cell migration. When studying the spread of tumors to secondary sites, it is therefore important to understand: (1 ) the response of tumor cell lines to motility boosting factors and (2 ) endothelial cell chemotaxis in response to tumor-derived angiogenic factors.