In the following pages we analyze the process of nutritional transition and its close relationship with the rising world-wide prevalence of obesity. We show that the changing eating patterns, as a consequence of globalization, are running in parallel with the alarming increase of overweight in both industrialised and developing countries. Thus, a uniform and opulent diet, together with a sedentary way of life typical of current societies, represent a first grade environment for the development of obesity. On the other hand it has to be analysed if distinct human groups have a different predisposition to accumulating fat as a result of recent evolution. In order to clarify this aspect and after revising the genetic basis of obesity, a special emphasis is given to the information available on bi-allelic SNP markers (Single Nucleotide Polimorphysms). As an example we describe a current research project.