Special issue of the International Journal of Sociology: “Interest in the use of “big data” in the social sciences is growing dramatically. Yet, adequate methodological research on what constitutes such data, and about their validity, is lacking. Scholars face both opportunities and challenges inherent in this new era of unprecedented quantification of information, including that related to political actions and attitudes. This special issue of the International Journal of Sociology addresses recent uses of “big data,” its multiple meanings, and the potential that this may have in building a stronger understanding of political behavior. We present a working definition of “big data” and summarize the major issues involved in their use. While the papers in this volume deal with various problems – how to integrate “big data” sources with cross-national survey research, the methodological challenges involved in building cross-national longitudinal network data of country memberships in international nongovernmental organizations, methods of detecting and correcting for source selection bias in event data derived from news and other online sources, the challenges and solutions to ex post harmonization of international social survey data – they share a common viewpoint. To make good on the substantive promise of “big data,” scholars need to engage with their inherent methodological problems. At this date, scholars are only beginning to identify and solve them….(More)”