Heparin is frequently used for treatment of cancer-associated thromboembolism. Accumulating clinical evidence indicates that cancer patients treated with unfractionated and low-molecular weight heparin survives longer that patients treated by other anticoagulants, especially patients in the early stage of a disease. Experimental analysis from a number of animal models constantly provides evidence about the ability of heparin to attenuate metastasis. The non-anticoagulant activity of heparin on metastasis includes the ability to inhibit cell-cell-interaction through blocking of P- and L-selectin, to inhibit extracellular matrix protease-heparanase, and to inhibit angiogenesis. This chapter summarizes the current experimental evidence on the biology of heparin during cancer progression with the focus on potential mechanism of heparin antimetastatic activity.