For a long time membrane biochemistry was almost synonymous with the bio­ chemistry of electron transport and oxidative phosphorylation. Although the successful analysis of hormone receptors, active transport, and other membrane-linked metabolic systems displaced mitochondria from the focus of interest, the field continued to grow and its contributions to other areas of membrane biochemistry played a major role in their dramatic development. The eight chapters in this volume dealing with electron transport provide a concise, critical, and up-to-date picture of the problems and accomplishments of the field. In the remainder of the volume a brief summary of selected receptor functions is presented. The relative novelty of this field naturally limits the factual scope of developments and encourages speculation. Nevertheless these reviews accurately reflect both accomplishments and deficiencies and provide objective guidance for future development. Several receptor functions omitted from these discussions will form one of the later volumes currently in preparation.