Protein unfolding is a key step in the import of some proteins into mitochondria and chloroplasts and in the degradation of regulatory proteins by ATP-dependent proteases. In contrast to protein folding, the reverse process has remained largely uninvestigated until now. This review discusses recent discoveries on the mechanism of protein unfolding during translocation into mitochondria. The mitochondria can actively unfold preproteins by unraveling them from the N-terminus. The central component of the mitochondrial import motor, the matrix heat shock protein 70, functions by both pulling and holding the preproteins.