In most organisms, the pyruvate dehydrogenase complex catalyzes the pivotal irreversible reaction that leads to the consumption of glucose in the aerobic, energy-generating pathways. A combination of biochemical and molecular biology studies have greatly expanded our understanding of the overall structural organization of this multicomponent system, delineated the locations and elucidated the functions of structural domains of the catalytic components, and revealed significant evolutionary changes. Important to this progress was the deduction of the primary amino acid sequences from cDNA clones for each of the catalytic components from several species. The greatest detail is available for the FAD-containing dihydrolipoamide dehydrogenase component, which is the only component for which tertiary structure information has recently emerged. For the dihydrolipoamide acetyltransferase core component, a similar but species-variable multidomain structure is established that is responsible for the distinct architectures of the inner cores, the peripheral binding of the other components, and the conveyance of reaction intermediates between distantly separated active sites. A second lipoyl-bearing component, protein X, has been shown to play a critical role in the organization and function of the complex from many higher organisms. Although much is known about the means of effector modulation of mammalian complex activity, identification of the signal eliciting its regulation by insulin still poses an exciting challenge.