Obesity is a highly prevalent chronic disease that carries enormous human and economical costs in Western nations. The early chapters of the book revealed that the prevalence of overweight and obesity was already quite high in Western societies and on the rise in developing countries of the world. The investigation of the association between DNA sequence variation at specific genes and obesity phenotypes has just begun. The limited molecular marker studies published so far suggest that there will likely be several genes associated and/or linked with human obesity. Recent progress in animal genetics, transfection systems, transgenic animal models, recombinant DNA technologies applied to positional cloning, and methods to identify loci contributing to quantitative traits have provided a new impetus to this field. An important contemporary issue is whether there are genotype-environment effects and/or gene-gene effects which influence the events leading to the various obesity phenotypes or to some of their outcomes.