The central role of the ribonucleic acids (RNA) in mediating the expression of information encoded in DNA in living cells is w 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 t 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.