It is the study of the role of genetic factors in determining the health and disease in families and in populations, and interplay of such genetic factors with environmental factors. Genetic Epidemiology is the scientific discipline that deals with the analysis of the familial distribution of traits, with a view to understanding any possible genetic basis. However, one cannot study genes except as they are expressed in people living in certain environments, and one cannot study environmental factors except as they affect people who have certain genotypes. Genetic Epidemiology is a uniquely interdisciplinary field that seeks to understand both the genetic and environmental factors and how they interact to produce various diseases and traits in humans. Human diseases have been the focal point of genetic epidemiologic studies and recent efforts are directed toward complex disorders such as coronary heart disease, hypertension, diabetes, obesity, cancer, atopy and allergies, and neurological and psychiatric disorders, to name a few. It is commonly thought that an understanding of the genetic underpinnings of such diseases will revolutionize medicine in the 21st century enabling better preventive measures, diagnosis, prognosis, and novel treatments.