Complex II (succinate:quinone oxidoreductase) of aerobic respiratory chains oxidizes succinate to fumarate and passes the electrons directly into the quinone pool. It serves as the only direct link between activity in the citric acid cycle and electron transport in the membrane. Finer details of these reactions and interactions are but poorly understood. However, complex II has extremely similar structural and catalytic properties to quinol:fumarate oxidoreductases of anaerobic organisms, for which X-ray structures have recently become available. These offer new insights into structure-function relationships of this class of flavoenzymes, including evidence favoring protein movement during catalysis.