Relationships between genes and diseases have long been hypothesized. The association of a disease with a gene dates back in Western medicine as far as Hippocrates, who hypothesized epilepsy was caused by a singular hereditary unit of biological material. However, with technological advances and the completion of the human genome sequence (1), scientists can now associate specific genetic variations with clinical conditions. Genetic associations provide informative clues for developing new diagnostic and therapeutic techniques to improve patient care. Understanding the principles that underlie genetic studies will become an essential skill for clinicians if we are to appreciate the complexity of genetic contributions to disease and its treatment (Table 5-1). © 2008 Springer-Verlag New York.