Cellular adhesion molecules are placed under force during the highly specific mechanical interactions that occur between proteins of the cell surface and extracellular matrix. In our laboratory we have developed atomic force microscopy (AFM) techniques to study the elastic properties of the extracellular matrix proteins at the single molecule level. We demonstrate that extracellular matrix proteins are elastic and that this elasticity involves reversible unfolding of single protein domains. Our data suggest that domain unfolding/refolding could be an important determinant of elasticity in extracellular matrix proteins and that the elastic properties of proteins could play role in cell-cell interactions. Furthermore, we have used protein engineering to construct tandem repeats of a single protein module and stretch it with the AFM. This strategy has allowed us to directly measure the unfolding and refolding rates of a single protein domain. We have compared these rates with chemical folding rates for untethered modules and found that the unfolding rates obtained by the two methods are the same. Our results demonstrate AFM as a unique tool to study the folding reactions of single protein domains and examine in unprecedented detail the molecular basis of protein elasticity.