Heparin is often used to prevent or treat thromboembolism in cancer patients. Clinical and experimental evidence suggest that heparin also has anticancer activities. Experimental evidence consistently supports the ability of heparin to attenuate metastasis. The potential antimetastatic effects of heparin include inhibition of cell-cell interactions and 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 on the basis of the insights obtained from experimental studies.