The extracellular matrix (ECM) holds cells together and maintains the three-dimensional structure of the body. It also plays critical roles in cell growth, differentiation, survival and motility. For a tumour cell to metastasize from the primary tumour to other organs, it must locally degrade ECM components that are the physical barriers for cell migration. The key enzymes responsible for ECM breakdown are matrix metalloproteinases (MMPs). To date, 23 MMP genes have been identified in humans and many are implicated in cancer. ECM degradation by MMPs not only enhances tumour invasion, but also affects tumour cell behaviour and leads to cancer progression. This review highlights recent developments with regard to the cellular and molecular mechanisms of MMPs that influence tumour cell growth, invasion and metastasis.