The Fourth Course of the International School of Pure and Applied Biostructure, a NATO Advanced Study Institute, was held September 18-31, 1983 at the Ettore Majorana Center for Scientific Culture in Erice, Sicily. The subject of the Fourth Course, which was co-sponsored by national and international agencies, was "Structure and Function of the Genetic "Appara­ tus. " Participants from 15 countries around the world attended the course. The study of the genetic apparatus is one of humanity's most challeng­ ing problems, and it has been approached in the tradition of the School from many different points of view, among them biochemistry, genetic eng­ ineering, cell biology, oncology, biophysics and other fields. It has been most difficult to confine such diverse points of view, as well as their pro­ ponents, within the four walls of one room, in front of one audience - es­ pecially since the heterogeneity of background and the inherent difficul­ ties encountered in communication could overshadow the true spirit of sci­ entific exchange. We are once again pleased to say the outcome of the 1983 Course has matched the success of the previous course held in Erice on the same subject five years ago. This book is the result of the 1983 Advanced Study Institute, and aims to present a cohesive, interdisciplinary view of the current knowledge on the structure and fuction of the genetic apparatus.