The first meeting of the NATO/CCMS Pilot Study "Dose-Response Analysis and Biologically-Based Risk assessment for Initiator and Promoter Carcinogens" was held in Rome, Italy, in the spring of 1991, and was followed by annual or bi-annual meetings held in Germany, Greece, Netherlands, Portugal, USA, up to the end of 1995; in large part supported by NATO/CCMS grants or fellowships, and organized by Pilot Study participants. The Pilot Study activity has been characterized by a higly collaborative atmosphere, which was essential for a deep and detailed analysis of a problem on which different points of view, methodological approaches and regulations exist in the various member countries. The Pilot Study was aimed at proposing a carcinogenic risk assessment procedure which is based on a detailed analysis of the relevant biological processes, and may also consent the verification of hypotheses. The specific form of theoretical and mathe­ matical models is identified by considering and using the whole set of objective data available. The multidisciplinary approach of the pilot study is reflected by the struc­ ture of this book. Each chapter is the result of the cooperation of several authors from to produce a comprehensive manual that includes different countries; its objective was both theoretical and practical information.