Matrix metalloproteinases (MMPs) are a family of zinc-containing enzymes involved in the degradation and remodeling of extracellular matrix proteins. Under normal physiological conditions, the activities of these enzymes are well-regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs). Chronic stimulation of MMP activities due to an imbalance in the levels of MMPs and TIMPs has been implicated in the pathogenesis of a variety of diseases such as cancer, osteoarthritis, and rheumatoid arthritis. Thus, MMP inhibitors are expected to be useful for the treatment of these disorders. Because of their importance in a variety of pathological conditions, a number of small molecular weight MMP inhibitors have entered clinical trials in humans. However, the results of these trials have been extremely disappointing and have led many investigators to conclude that MMP inhibitors have no therapeutic benefit in human cancer. To date, the first generation MMP inhibitors exhibited poor bioavailability while second-generation compounds revealed that prolonged treatment caused musculoskeletal pain and inflammation or had a lack of efficacy. This article describes the design of small molecular weight MMP inhibitors, a brief description of available three-dimensional MMP structures, a review of the proposed therapeutic utility of MMP inhibitors, and a clinical update of compounds that have entered clinical trials in humans. The experimentally determined structures used in the structure-based design of MMP inhibitors are thoroughly covered. Major emphasis is on recently published and / or patented potent MMP inhibitors, from approximately January 2000 to April 2003, and their pharmacological properties. Protein inhibitors of these proteolytic enzymes, i.e. TIMPs, will not be discussed.