Mitochondria import most of their proteins from the cytosol. Although considerable information is available on the import machineries of the mitochondrial outer membrane and matrix, until recently little was known about the machinery of the inner membrane. Recent studies have identified three mitochondrial inner membrane proteins (MIMs) as essential components of the import machinery. MIM17 and MIM23 seem to form part of a channel, while MIM44, in cooperation with the heat-shock protein Hsp70, binds the preproteins in transit. The electrical membrane potential and ATP are needed to drive protein translocation through the MIM import machinery.