Fibronectin (FN) is a large modular glycoprotein that is found as a polymeric fibrillar network in the extracellular matrix (ECM) and as soluble disulfide-linked dimeric protomers in plasma and other body fluids. Fibronectin is a ligand for many molecules including fibrin, heparin, chondroitin sulfate, collagen/gelatin, and integrins. It is involved in multiple cellular processes such as cell adhesion/migration, blood clotting, morphogenesis, tissue repair, and cell signaling. Fibronectin functions are mediated by the insoluble polymeric fibrils. Conversion of soluble fibronectin to fibronectin fibrils in the ECM is initiated by binding to cell surface integrins, resulting in exposure of cryptic epitopes necessary for polymerization.