Matrix metalloproteinases (MMPs) are proteolytic enzymes which play a major role in tumour invasion. They are mainly produced by host stromal cells in most carcinomas and their expression implies a close co-operation between tumour and stromal cells. Increasing data also demonstrate that, in association with a process of epithelial-to-mesenchymal transition, many MMPs can be expressed by tumour cell themselves. Their most well-known role is the degradation of extra-cellular matrix macromolecules which in turn may regulate tumour invasion in some conditions. This ECM degradation generates some matrikins which are also implicated in tumour invasion and angiogenesis. Moreover, MMPs are also implicated in the degradation of cell adhesion molecules and release and activation of growth factors.