Heparin is often used to prevent or treat thromboembolism in cancer patients. Clinical and experimental evidence suggest that heparin also has anti-cancer activities. Experimental evidence consistently supports the ability of heparin to attenuate metastasis. The potential anti-metastatic effects of heparin include the inhibition of cell-cell interactions or heparanase and modulation of growth factors and anticoagulant activity. Heparin inhibits selectin-mediated interactions of tumor cells with leukocytes, platelets and endothelial cells, which are likely to mediate the initial steps of hematogenous metastasis. Prospective clinical trials can be designed based on the insights obtained from experimental studies.