About 2.5 billion adults, just over half the world's adult population, lack bank accounts. If we are to realize the goal of extending banking and other financial services to this vast "unbanked" population, we need to consider not only such product innovations as microfinance and mobile banking but also issues of data accuracy, impact assessment, risk mitigation, technology adaptation, financial literacy, and local context. In Banking the World, experts take up these topics, reporting on new research that will guide both policy makers and scholars in a broader push to extend financial markets. The contributors consider such topics as the complexity of surveying people about their use of financial services; evidence of the impact of financial services on income; the occasional negative effects of financial services on poor households, including disincentives to work and overindebtedness; and tools for improving access such as nontraditional credit scores, financial incentives for banking, and identification technologies that can dramatically reduce loan default rates.