Abstract: Mitochondria import most of their proteins from the cytosol. Dynamic protein complexes in the mitochondrial outer and inner membranes are responsible for the specific recognition and membrane translocation of preproteins. The preprotein translocase of the outer mitochondrial membrane contains several import receptors and a general import pore. The preprotein translocase of the inner membrane consists of a channel interacting with preproteins in transit and an import motor that includes the matrix heat shock protein Hsp70. Acidic patches of import components are thought to guide the import of positively charged signal sequences (acid chain hypothesis). Energy input is derived from the inner membrane potential and ATP. Proteins in the mitochondrial matrix are required for proteolytic processing and folding of imported proteins. The dynamic nature of the membrane translocase permits sorting of preproteins at distinct stages of the import pathway.