The invasion and metastasis of cancer cells is a complex multistep process involving destruction of basement membranes as an early event in the metastatic cascade. Recent evidence implicates secreted matrix metalloproteinase enzymes, such as type IV collagenases, as playing a central role in this tumor cell mediated extracellular matrix proteolysis. Two distinct type IV collagenase enzymes are now recognized. Immunohistochemical and biochemical studies of several human tumors show correlations between invasive potential and the 72 kDa type IV collagenase enzyme. Studies in rodent tumor models suggest that the 92 kDa type IV collagenase may play an important role in these models, but data on human tumors and human tumor tissue is lacking. Evidence suggest that the regulation of the 72 kDa type IV collagenase enzyme activity may occur at many levels, including transcriptional mechanisms, extracellular activation of latent enzyme and specific inhibitors of active enzyme. Thus the invasion of human tumor cells through basement membranes may be the result of net type IV collagenolytic activity that is the result of a balance of activated enzyme species and inhibitors.