The extracellular matrix protein fibronectin has important roles in cell adhesion, migration, growth and differentiation. It is secreted as a soluble dimer but subsequently assembles to form insoluble, multimeric fibrils. The assembly process depends on interactions between fibronectin and cell surface integrins and is regulated by several signalling pathways. In a Commentary on p. 3269, Iwona Wierzbicka-Patynowski and Jean Schwarzbauer discuss work that is shedding light on this complex mechanism. It is usually initiated by binding ofα5β1 integrin to the RGD motif of fibronectin. Since fibronectin is a dimer, this promotes integrin clustering, fibronectin-fibronectin interactions and formation of fibrils. Another consequence of integrin binding is an opening up of the fibronectin molecule to expose hidden multimerization motifs. Recent studies indicate that partial unfolding of the molecule imparts elasticity on fibrils and allows them to remain under tension. This could increase the pliability of the matrix and is probably regulated by signals that control actomyosin contractility. Other signalling molecules that regulate fibronectin assembly include FAK and Src; these tyrosine kinases lie downstream of integrins and appear to participate in feedback loops that target the fibronectin ligand. Indeed perturbation of fibronectin assembly by Src and other oncogene products probably has a significant impact on tumor cell phenotype.