The matrix metalloproteinases (MMPs) have been associated with tissue remodeling in many normal and pathological processes, and in particular are thought to be critical for tumor invasion and metastasis. MMP overexpression has been correlated with the stage of progression in several tumor types. Because of the aberrant nature of tumor cells, it has been assumed that the tumor cells themselves are responsible for this abnormal MMP production. However, recent in situ hybridization experiments have determined that, while some members of the MMP family are expressed within neoplastic cells, many are found in normal stromal components immediately adjacent to the tumor tissue. In this review, we address the potential roles of both tumor and stromal MMPs in tumor progression. Also, using MMP expression as a point of reference, we discuss some of the potential mechanisms involved in tumor/host interactions.