The main difference between a benign and malignant tumor is the ability of the malignant form to invade normal tissue and spread or metastasize to distant sites throughout the body. It is the ability to form metastasis which makes cancer such a difficult disease to treat. Evidence suggesting that proteolytic enzymes are involved in cancer spread is as follows: proteases are involved in normal destructive events and tissue remodelling, correlations exist between different protease activities and metastatic potential in model tumor systems, inhibitors and antibodies against proteases inhibit metastasis in model systems and the finding of highest levels of protease activity at the invading front in tumors. The most likely mechanism by which proteases could mediate metastasis is by catalyzing degradation of the extracellular matrix and basement membranes. It is concluded that if proteases could be proved to play a role in the spread of human cancers, inhibition of these enzymes could open up new therapeutic approaches for the control of malignancy.