Mitochondria are ubiquitous, double-membrane bound organelles, which have developed from endosymbiotic alpha-proteobacteria during evolution. Outer and inner membranes of mitochondria are equipped with characteristic sets of membrane proteins required for energy conversion, metabolite and protein transport, membrane fusion and fission, and signal transduction. Mitochondrial membrane proteins are encoded by both, the nuclear and the mitochondrial genomes, and exhibit divergent transmembrane topologies. Correct targeting and membrane integration of these proteins and subsequent assembly into functional protein complexes must be tightly coordinated. This elaborate task is mediated by the cooperative functions of different protein import and export machineries of the outer and inner mitochondrial membranes.