The fact that about 30-40% of the adults in the Western world are overweight or obese testifies to the frequency of the disturbances in body weight regulation. Scientists have established that caloric intake, macronutrient composition of the diet, basal and resting metabolic rate, thermic response to food, energy expenditure associated with movement and physical activity, and preferential storage of the surplus of calories as fat or lean tissues are critical determinants of energy balance and body weight. While much has been learned, the field is poised for major advances with the advent of a variety of imaging techniques, progress in quantitative and molecular genetics, use of transgenic rodent models and of breeding experiments with informative inbred strains, availability of stable isotopes for metabolic and behavioral studies, and a growing number of useful experimental animal and human models. This volume takes an integrative approach to obesity. It is structured around four major topics: (1) the animal and human models currently available for the study of body weight regulation with their strengths and limitations, (2) the molecular and genetic basis of the regulation of body weight, (3) the metabolic and physiological mechanisms involved, and (4) the behavioral and social determinants. The 13 background papers provide a critical overview of the present knowledge base while the group reports summarize the extensive deliberations of 38 international experts. Particular emphasis has been given to promising research areas and on the advances needed to ensure a better understanding of the biological and behavioral mechanisms of the regulation of body weight, with a particular emphasis on overweight and obesity.