The family of human matrix metalloproteinases (MMPs) comprises several tightly regulated classes of proteases. These enzymes and their specific inhibitors play important roles in tumor progression and the metastatic process by facilitating extracellular matrix (ECM) degradation. As scientific understanding of the MMPs has advanced, therapeutic strategies focusing on blocking these enzymes by MMP inhibitors (MMPIs) have rapidly developed. This paper reviews MMPs in detail. Their perspectives in therapeutic intervention in cancer are also mentioned.