The central role of the ribonucleic acids (RNA) in mediating the expression of information encoded in DNA in living cells is now well established. Research in this area of biology continues at a remarkable rate, and new and significant information appears almost daily in a wide range of journals, published symposia and specialist reviews. The diverse nature of this information makes it difficult for the newcomer to the field of RNA biochemistry to obtain a general view of established concepts, current activity, and new advances. Moreover, the reviews available are frequently concerned with insular aspects of these Ubiquitous molecules, or in the case of text­ books, the subject is treated as part of a general outline of proper­ ties of nucleic acids and thus may be superficial. The authors of the chapters in this collection attempt to provide a comprehensive, though not overly detailed, outline of the biologi­ cal roles of RNA. They have written for students with basic training in biochemistry, but otherwise with a wide variety of biological interests-plant physiology, virology, organelle bio­ chemistry, genetics, cell biology, differentiation and development. Viral RNA, which was dealt with as a separate chapter in the first edition, has been deleted from this edition because it is an unman­ ageably large single topic, and at the same time is addressed in a number of ways in many different places in the book.