Airway remodeling is a major change responsible for irreversible asthmatic airflow restriction. The Th-2 cytokines-dominant eosinophilic inflammatory mechanism cannot fully explain the progressive subepithelial fibrosis and structural changes in the extracellular matrix (ECM). Matrix metalloproteinases (MMPs) are the key enzymes responsible for ECM degradation. MMPs are normally produced and secreted under the tight regulation of, at least, 3 different levels: the gene transcriptional level, the activation of the latent form of enzyme, and the inactivation by specific endogenous inhibitors. In asthmatic condition, as shown by the large amount of accumulated evidence in this review, MMP-9 is the most relevant among the 23 kinds of human MMPs at present detected. Although the mechanism is still under investigation and not accurately known, the imbalance between MMP-9 and tissue inhibitor of metalloproteinase-1 is considered a major theory to explain the progression of asthmatic airway remodeling. Various inflammatory cytokines including TGF beta and growth factors play a pivotal role in MMP-9 production and secretion. This review mainly focuses upon the pivotal role of MMP-9 in airway remodeling, and also upon major cellular source of MMP-9 in asthma such as eosinophils, neutrophils, epithelial cells and alveolar macrophages. This review also refers to the partial contribution of nitric oxide to MMP-9 in asthma.