This chapter presents the main questions raised in this book, as well as introducing some key notions that play a role in their answering. How did languages spread across the globe, and what are the implications for our understanding of human prehistory? Sometimes we find large language families, and elsewhere clusters of very small families or language isolates. What was the role of agriculture in language spread, and what did different language ideologies and patterns of ethnic identity formation contribute? Do geography and climate help explain dispersal patterns? Notions discussed are diversity versus disparity and the role of different time depths, followed by a survey of different continents. The models of dispersal in the work of Nichols, Dixon, and Nettle are analysed and contrasted, and the Farming Dispersal Hypothesis critically reviewed. Then the concepts of language families, linguistic areas, and linguistic isolates are presented, followed by a discussion of diversification mechanisms.