The cytochrome bc1 complex (bc1) is the mid-segment of the cellular respiratory chain of mitochondria and many aerobic prokaryotic organisms; it is also part of the photosynthetic apparatus of non-oxygenic purple bacteria. The bc1 complex catalyzes the reaction of transferring electrons from the low potential substrate ubiquinol to high potential cytochrome c. Concomitantly, bc1 translocates protons across the membrane, contributing to the proton-motive force essential for a variety of cellular activities such as ATP synthesis. Structural investigations of bc1 have been exceedingly successful, yielding atomic resolution structures of bc1 from various organisms and trapped in different reaction intermediates. These structures have confirmed and unified results of decades of experiments and have contributed to our understanding of the mechanism of bc1 functions as well as its inactivation by respiratory inhibitors. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes. ► Crystal structures of cyt bc1 complexes have been determined from diverse organisms. ► ISP-ED, the extrinsic domain of iron-sulfur protein, undergoes binary conformational changes upon binding of different inhibitors. ► ISP-ED conformation is controlled through modulating its binding affinity to cyt b subunit. ► Bifurcated ET can be explained by the “surface-affinity modulated ISP motion switch hypothesis”. ► This mechanism has received substantial experimental support.