The action of matrix metalloproteinases (MMPs) was originally believed to be restricted to degradation of the extracellular matrix; however, in recent years, it has become evident that these proteases can modify many nonmatrix substrates, such as cytokines and chemokines. The use of MMP-deficient animals has revealed that these proteases can indeed influence the progression of various inflammatory processes. This review aims to provide the reader with a concise overview of these novel MMP functions in relation to leukocyte migration.