Protein unfolding is an important step in several cellular processes, most interestingly protein degradation by ATP-dependent proteases and protein translocation across some membranes. Unfolding can be catalyzed when the unfoldases change the unfolding pathway of substrate proteins by pulling at their polypeptide chains. The resistance of a protein to unraveling during these processes is not determined by the protein's stability against global unfolding, as measured by temperature or solvent denaturation in vitro. Instead, resistance to unfolding is determined by the local structure that the unfoldase encounters first as it follows the substrate's polypeptide chain from the targeting signal. As unfolding is a necessary step in protein degradation and translocation, the susceptibility to unfolding of substrate proteins contributes to the specificity of these important cellular processes.