β-barrel proteins are present in the outer membrane of Gram-negative bacteria and of organel- les of endosymbiotic origin, i.e. mitochondria and chloroplasts where they perform a variety of functions. Mitochondrial β-barrel proteins are important for protein import, metabolite transport and the organelle morphology and distribution. They also seem to play a crucial role in mitochondria evolution. Quite recently a specific pathway for the insertion of β-barrel proteins was identified in both mitochondria and Gram-negative bacteria and was proved to be conserved during evolution. In mitochondria the pathway is formed by the TOB/SAM complex (topogenesis of the mitochondrial outer membrane β-barrel proteins/ sorting and assembly machinery) composed of three main proteins, namely Tob55 (Sam50), Tob38 (Sam35) and Mas 37 (Sam37). Phylogenetic analysis provides a strong case for the evolution of Tob55 from the bacterial homologue Omp85 while other mitochondrial β-barrel proteins do not display amino acid homology with bacterial β-barrel proteins.