Epidemiology has been defined as “the study of the distribution and determinants of disease frequency in human populations” . Its inherent assumption is that through systematic investigation of different populations or subpopulations in different places or at different times, causal and preventive factors of human disease can be identified. Thus, an essential task in epidemiology is first to quantify the occurrence of illness, i.e. its frequency and distribution within populations. Secondly, the goal is to evaluate hypotheses about the causation of disease by measuring the effects of people’s characteristics and their environment on the occurrence of illness. Thirdly, epidemiology has great potential to improve the understanding of disease mechanisms by longitudinally observing and meticulously describing different phenotypes of chronic diseases and their natural course from infancy over childhood and adolescence in adulthood. The advent of modern genetic tools has created a new field of genetic epidemiology, which attempts to tackle the many hurdles in deciphering the role of a subject’s genetic makeup in the development of chronic, multifactorial disease.