Heparin, which is traditionally used as an anticoagulant but has a variety of additional biological activities, was shown in several retrospective and prospective clinical trials to have an effect on cancer survival. Experimental evidence from animal models consistently demonstrates that heparin is an efficient inhibitor of metastasis. To clarify the mechanism of heparin antimetastatic activity, several biological effects are being investigated. Cancer progression and metastasis are associated with enhanced expression of heparanase, which is inhibited efficiently by heparin. Heparin is also a potent inhibitor of selectin-mediated interactions. P- and L-selectin were shown to contribute to the early stages of metastasis, which is associated with platelet-tumor cell thrombi formation. To delineate the biological activities of heparin contributing to metastasis inhibition, modified heparins with specific activities were evaluated. Low anticoagulant heparin preparations still inhibited metastasis efficiently, indicating that anticoagulation is not a necessary component for heparin attenuation of metastasis. Modified heparins characterized for heparanase inhibitory activity are also potential inhibitors of selectins. Selectin inhibition is a clear component of heparin inhibition of metastasis. The contribution of selectin or heparanase inhibition by heparin can provide evidence about its antimetastatic activity.