Modern laboratory and computing advances have made it possible to identify which genes are responsible for a disease (or other biological traits) and to identify those genes. This book presents a survey of the methods that are being used to generate these successes, especially to study disease in families. The methods of epidemiology and genetics are surveyed, and related to molecular genetic data, with examples from both pediatric and chronic disease. The pattern of variation that has been found is best understood from the evolutionary perspective. Because these methods and ideas apply to any biological trait, not just to disease, this is a general book about the genetic control of biological traits.