The proceedings of the NATO Advanced Research Workshop on Biological Electron Transfer Chains contains reports on three themes. (a) Electron transfer: The analysis of ET at the molecular level is still fundamental to an understanding of how ET chains operate in vivo. After 40 years of research, the contours of the subject are now becoming clear. (b) Bacterial redox chains: These contributions show how complicated these chains can be, often involving a number of gene clusters, each of which in turn consists of an extensive array of genes. Our understanding of the regulatory aspects of these chains is only just beginning to emerge. (c) Structure and function of redox proteins: Structural information on proteins remains indispensable to a proper understanding of their function and of the context in which they operate. The two final chapters deal with oxido-reductases and the cytochrome oxidase family.