This book brings together various contributions aimed at the elucida­ tion of the structural and functional organization of the bacterial nucleoid. Most of these papers, spanning the fields of physical chemistry through biochemistry to genetics, were presented at the session on bac­ terial chromatin during the Symposium "Selected topics on chromatin structure and function" held at the University of Camerino, Italy, at the end of May 1985. Times when the bacterial DNA was regarded as "naked" or, at most, complexed with polyamines, and when the absence of histones and organized chromatin was considered to be a distinct feature of the pro­ karyotic cell, now appear remote. Our concepts of how DNA is packaged in bacteria are changing rapidly. Studies on the structure of the bacterial nucleoid are not new. Recently, however, investigations in this field have flourished again, leading to some important contributions such as the elucidation of the three-dimensional structure of what appears to be the major protein constituent of the bacterial nucleoid or the development of methods to titrate the extent of DNA supercoiling within the bacterial cell.