As mobile phones reach the remote corners of the world, they bring with them a sense of great optimism. Hailed as a technology that "can transform the lives of the people who are able to access them," mobile phones have the potential to play a positive role in the lives of many of the world's poor. Such claims are often reported alongside striking statistics on the uptake of mobile phones in the developing world. Already, over two thirds of the world's mobile phones are in developing countries. In Nigeria, new subscribers are signing up for mobile phone services at a rate of almost one every second, and Nokia estimates that by the end of 2012 over 90 percent of sub-Saharan Africa will have mobile coverage. This dissertation presents an empirical investigation of the role of mobile phones in Rwandan society and economy. The material draws on two summers of field work in sub-Saharan Africa, several thousand interviews with mobile phone owners, and roughly ten terabytes of data on mobile phone use that I obtained from Rwanda's largest telecommunications operator. In the first chapter, I analyze the distribution of mobile technology within the Rwandan population, drawing attention to disparities in access to and use of mobile phones between rich and poor, and between men and women. The analysis highlights three sets of results. First, comparing the population of mobile phone owners to the general Rwandan population, I find that phone owners are considerably wealthier, better educated, and more predominantly male. Second, based on self-reported data, I observe statistically significant differences between genders in phone access and use; for instance, women are more likely to use shared phones than men. Finally, analyzing the complete call records of each subscriber, I note large disparities in patterns of phone use and in the structure of social networks by socioeconomic status. The second chapter focuses on the economic implications of the spread of an early form of "mobile money" in Rwanda, and provides empirical evidence that this electronic currency is used to transmit funds to individuals affected by catastrophic shocks. Contrasting two stylized models of prosocial behavior, this analysis provides insight into why people help each other in times of dire need. The findings are based on the analysis of interpersonal interactions occurring immediately before and after a destructive earthquake in Rwanda. The observed pattern of transfers is not consistent with a model of pure charity or altruism, but better fits a model of risk sharing in which individuals mutually insure each other against uncorrelated income shocks. The third and fourth chapters present methodological contributions, and serve to illustrate how mobile phone data can be used to observe and understand the behavior of populations in developing countries, at a level of detail typically unobserved by social scientists. Chapter 3 develops a method for measuring levels and patterns of internal migration. After formalizing the concept of inferred mobility, I compute this and other metrics for 1.5 million Rwandans, and provide novel quantitative evidence consistent with qualitative findings by other scholars. Chapter 4 describes a new method for using mobile phone data to predict the socioeconomic status of an individual. The approach uses mixed methods and three distinct sources of data: anonymous call records; a government Living Standards and Measurement Survey; and a set of phone surveys I conducted in 2009 and 2010. The chapters in this dissertation develop theory and methods for understanding how mobile technologies influence economic and social behavior, and how new sources of data can be used to provide insight into patterns of human interaction. Taken together, the empirical results indicate that phones have had a positive impact on the lives of some people but, absent intervention, the benefits may not reach those with the greatest need. The ultimate goal of these studies is to better understand how information and communications technologies are changing, and can be used to improve, the lives of people worldwide.