Six years ago when the first edition of this volume appeared as the first of the series, questions were posed in its preface that are as valid today as they were then. In that preface, I proposed the following challenges: (1) We must identify carcinogenic agents, and by an analysis of their "nature," e. g., structure and physical characteristics, we may better understand their mechanism of action. (2) We must identify crucial interactions between these carcinogens and impor- tant macromolecules within the cell, distinguishing those which relate to carcinogenesis from those which are...