The use of historical data has become a standard tool in economics, serving three main purposes: to examine the influence of the past on current economic outcomes; to use unique natural experiments to test modern economic theories; and to use modern economic theories to refine our understanding of important historical events. In this chapter, we provide a comprehensive analysis of the types of historical data most commonly used in economic research and discuss a variety of issues that they raise, such as the constant change in national and administrative borders; the reshuffling of ethnic groups due to migration, colonialism, natural disasters, and many other forces. We also point out which methodological advances allow economists to overcome or minimize these problems.