Periostin, also known as OSF-2, is a secreted matricellular protein with functions in extracellular matrix formation, cell migration, and inflammation. It is secreted as a 90 kDa monomer that can aggregate into >170 kDa higher-order multimers. Periostin is expressed by mesenchymal cells such as vascular smooth muscle cells, fibroblasts, osteoblasts, and odontoblasts in developing teeth. It is upregulated in many carcinomas. Periostin binds to multiple Integrins and enhances cell adhesion and cell migration. It enhances Fibronectin and Collagen I production and promotes collagen fibrillogenesis. It also induces epithelial-mesenchymal transition, tumor growth, invasion, and metastasis. Periostin induces the expression of VEGF R2 on endothelial cells and VEGF-C in tumor cells, and it can induce tumor lymphangiogenesis. Periostin plays an important role in heart valve development and tissue healing after myocardial infarction. In asthma, it is upregulated in bronchial epithelium and plays both destructive and protective roles by inducing eosinophil infiltration and inhibiting goblet cell metaplasia and mucus production.