From May 3-7,1997, the NATO Advanced Research Workshop on 'Biological Electron Transfer Chains' was organized in Tomar, Portugal. In the application for support the choice of the topic was justified as follows: "[Until recently efforts] have concentrated on the study of the structure and function of individual redox enzymes and proteins. Enough information is now available to make a start with the study of biological electron transfer (E1) at the next higher level of organization, that of the complete ET chain." The interest in the workshop was high: the majority of participants had registered before the workshop was formally announced, which illustrates the popularity of the topic within the biochemical and biophysical communities. The present volume contains a number of reports based on the lectures presented by the key speakers during the meeting. The workshop dealt with the following three themes: a) Electron transfer, which is the subject of Chapter 1. The analysis of ET at the molecular level is still fundamental for an understanding of how ET chains operate in vivo. After 40 years of research the contours of the subject are becoming clear now. b) Bacterial redox chains. This is the subject of Chapter 2. Its contents show how complicated these chains can be, often involving a number of gene clusters. Our understanding of the regulatory aspects and control mechanisms of these chains is only in its beginning.