In the last years the picture of protein import into the mitochondria has become much more complicated in terms of new components and new sorting pathways. These novel findings have also changed views concerning the biogenesis pathway of mitochondrial outer membrane proteins. In addition to proteins anchored with transmembrane alpha-helices, the endosymbiotic origin of the mitochondria has resulted in the presence of transmembrane beta-barrels in this compartment. The sorting and assembly pathway of outer membrane proteins involves three machineries: the translocase of the outer membrane (TOM complex) the sorting and assembly machinery (SAM complex) and the MDM complex (mitochondrial distribution and morphology). Here we review recent developments on the biogenesis pathways of outer membrane proteins with a focus on Tom proteins, the most intensively studied class of these precursor proteins.