This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book explores the relationship between DNA and history and describes the search for human history using genetic data. It also explores the peopling of the Americas and the genetic histories of Native Americans, the first humans to live in North and South America. The book provides questions about population movements but focuses on a case in which humans moved comparatively recently into a previously uninhabited region, the Pacific Islands. It examines the relationship between genetics and history in a single nation: Ireland, the focus of John's early professional career. The book explains genetic admixture, or the genetic exchange that occurs when populations come into contact after having previously been separated by time or distance. It assesses the genetic consequences of the Jewish Diaspora.