Fibronectin (FN) matrix fibrils assembled in cell culture have been observed to stretch in response to cell movements, and when broken relax to 1/3 to 1/4 of their rest length. Two molecular mechanisms have been proposed, for the elasticity. One proposes that FN molecules in relaxed fibers are bent and looped into a compact conformation, and stretching pulls the molecules into the extended conformation but domains remain folded. The second proposes that molecules in fibrils are already extended, and stretching is produced by force-induced unfolding of FN type III domains. Experimental observations that may help distinguish these two possibilities are discussed.