Susceptibility to most common human diseases is, at least in part, determined by genetic factors. Rapid progress is being made in defining these genetic determinants for a range of diseases including breast cancer, colon cancer, diabetes, arthritis and dementia. The ability to define susceptibility in genetic terms has already led to a reclassification of some of these diseases on genetic and mechanistic grounds. This information is likely to have a profound effect on our approach to human diseases as it will allow a better definition of these disorders, permitting more effective therapeutic intervention, and will lead to both a more precise understanding of the natural history of these diseases and the possibility of identifying populations at risk. An understanding of the mechanisms underlying disease susceptibilty will also improve our ability to develop rational therapeutic interventions for many of these diseases. The role of genetic screening in these common diseases will be discussed, particularly in regard to the application of health care in populations.