Pulmonary fibrosis, the final result of a large variety of interstitial lung diseases, is characterized by an aberrant remodeling of extracellular matrix (ECM) with a profound disturbance of the normal lung architecture. This remodeling includes the exaggerated accumulation of ECM components in the interstitial and alveolar spaces and the disruption of the basement membranes. It has long been accepted that matrix metalloproteases (MMPs) play an important role in the pathogenesis of pulmonary fibrosis, but the exact mechanisms are not well characterized. Several MMPs are strongly up-regulated in human and experimental lung fibrosis, highlighting the dynamic nature of scarring within the lung. MMPs are collectively capable of cleaving all components of the ECM and basement membranes, but importantly, they also process bioactive mediators such as growth factors, cytokines, chemokines, and cell-surface receptors. Moreover, they participate in the initiation of proteinase cascades that impact much broader substrates. Consequently, MMPs may play a central role in several interrelated processes observed in fibrosis such as ECM remodeling, basement-membrane breakdown, epithelial-cell apoptosis, cell migration, and angiogenesis.