In this volume of SUBCELLULAR BIOCHEMISTRY we cover a wide range of topics of considerable biological importance and have continued in our policy of letting authors, rather than editors, decide the "natural" length of their articles. Thus, we have some short but nevertheless significant contributions, as well as more massive chapters. We start with a detailed account by 1. Oelze of the composition and development of the bacterial photosynthetic apparatus. A number of photosynthetic bacteria are discussed, with particular emphasis on the well-studied Rhodospirillum rubrum and Rhodopseudomonas sphae­ roides. The reader will no doubt be struck by the great wealth of information now available on the molecular organization of the photosynthetic and respi­ ratory systems in these organisms. Equally important is our improved understanding of the biosynthesis and assembly of these systems. It is now generally accepted that photosynthetic bacteria are excellent model systems for the study of bioenergetic processes. It may well be that they will become equally popular as models for the study of membrane biogenesis, and it is to be hoped that Oelze's erudite and comprehensive treatment of the subject will help in this regard.