Mitochondria are essential organelles. As the mitochondrial genome encodes only a few proteins, the vast majority of proteins that contribute to mitochondrial function are translated in the cytosol from nuclear-encoded transcripts and must be selectively imported into the organelle. The specificity of protein import is achieved by the translocase of the outer mitochondrial membrane (TOM) complex: a multisubunit receptor complex and translocation pore. Further molecular machines including the sorting and assembly machinery (SAM) complex of the mitochondrial outer membrane then transfer and assemble proteins into one of the four mitochondrial subcompartments. Both the TOM and SAM complexes exhibit a modular structure. The core TOM complex is composed from the most highly conserved subunits, and is complemented by more recently evolved add-on modules, such as the receptors Tom20 and Tom70. These new modules provide sophistication to the complex, enhancing the specificity and capacity for binding substrates. Likewise, a core SAM complex is critical for the assembly of β-barrel proteins into the outer mitochondrial membrane. Additional modules such as the metaxin-type proteins Sam37 and Sam35 and the yeast proteins Mim1 and Mdm10 are required for the assembly of some, but not all, SAM substrates. Mdm10 is an excellent example of a modular component, being found in at least one other complex where it assists with mitochondrial distribution and morphology. The concept of modular design provides an interesting new perspective on functional aspects of the protein import machinery in mitochondria.