[en] The cytochrome bc complexes represent a phylogenetically diverse group of electron-transfer membrane protein complexes, most familiarly represented by the mitochondrial and bacterial bc1 complexes and the chloroplast and cyanobacterial b6f complex. All these complexes couple electron transfer to protein translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past 5 years of x-ray structures for several mitochondrial complexes, and for two important domains of the b6f complex