The unprecedented growth of data and its ubiquity in our daily lives signal that the digital revolution is transforming the world. But much of the value of data remains untapped, waiting to be realized. Data collected for one particular purpose has the potential to generate economic value in applications far beyond those originally anticipated. However, many barriers stand in the way of the beneficial reuse of data, ranging from misaligned incentives and incompatible data systems to a fundamental lack of trust. The World Development Report 2021: Data for Better Lives will explore the tremendous potential of the changing data landscape to improve the lives of poor people, but also to open backdoors that can harm individuals, businesses, and societies. The first part of the report assesses how better use and reuse of data can enhance the design of public policies, programs, and service delivery, as well as improve market efficiency and job creation through private sector growth. The second part of the report focuses on issues of governance, law, and policy that can help realize data's potential benefits while safeguarding against harmful outcomes. By examining these issues, the report aims to show how data can be leveraged to benefit the lives of poor people.