Matrix metalloproteinases (MMPs) are a family of zinc-dependent proteinases that are associated with the tumorigenic process. MMPs degrade the extracellular matrix, promoting tumor invasion and metastasis. They also regulate host defense mechanisms and normal cell function; blocking all MMPs may not lead to a positive therapeutic outcome. Most clinical trials of MMP inhibitors (MMPIs) have yielded disappointing results, perhaps due to inappropriate study design or tumor staging, or to lack of selectivity. Positive results have been seen in gastric cancer with marimastat and in Kaposi's sarcoma with metastat. This review summarizes the current status of MMPIs.