This book addresses the complex question of how and why languages have spread across the globe: why do we find large language families distributed over a wide area in some regions, while elsewhere we find clusters of very small families or language isolates? What roles have agriculture, geography, climate, ethnic identity, and language ideologies played in language spread? In this volume, international experts in the field provide new answers to these and related questions, drawing on the increasingly large databases available and on novel analytical research techniques.The first part of the volume outlines some general issues and approaches in the study of language dispersal, diversification, and contact. In the rest of the volume, chapters compare the language and population histories of three major regions - Island Southeast Asia/Oceania, Africa, and South America - which show particularly interesting contrasts in the distribution of languages and language families. The volume is interdisciplinary in approach, with insights from archaeology, genetics, anthropology, and geography, and will be of interest to a wide range of scholars interested in language diversity and contact.