Matrix metalloproteinases (MMPs) have been implicated in the invasive growth and spread of tumours. Potent and selective inhibitors of MMPs have been synthesized. These inhibitors can prevent tumour invasion in vitro and have suppressed tumour growth and metastasis in animal cancer models. Several MMP inhibitors have now reached clinical trials. Phase I studies indicate that the inhibitors can be given by mouth and that over the short term they are well tolerated. Longer, randomised studies in cancer patients are now underway.