There are currently a growing number of laboratories actively studying the mechanism by which various biological membranes are assembled. This area of research is still relatively new to biochemists and molecular biologists, but in view of the rapid progress being made, a review of the field at this time is justified. The present volume focuses on the biogenesis of three related membranes. Mitochondria and chloroplasts are semiautonomous organelles whose biogenesis is carried out partly in the external cytoplasm and partly by the organelles themselves. Both membranes are principally concerned with the energy metabolism of the cell, and this commonality of function is reflected in a considerable degree of similarity in their ul­ trastructure and enzymatic composition. Although the bacterial cell membrane is a much more diversified structure, it also fulfills the basic energy requirements of the cell, and depending on the organism, this can take the form of photosynthesis or oxidative phosphorylation. The addi­ tional consideration that prokaryotic organisms may, in fact, be the evolu­ tionary ancestors of mitochondria and chloroplasts, makes it all the more compelling that those interested in biogenesis be aware of new develop­ ments in each of these three areas. In organizing this book, I felt that the contributors should summarize and bring up to date their own research and review the literature only in­ sofar as would be necessary to provide the proper perspective for their work.