The aim of this book is to explain how web data and digital traces are revolutionising social sciences and providing new research questions, new methods and new insights. The digital revolution has not only changed the way individuals search for information, work, interact with friends or other people, and consume goods and services, but also generated new sources of data that allow social scientists to revisit traditional questions such as social influence, collective action and political engagement, as well as to address new research questions. Through the Internet, researchers potentially have access to massive amounts of data and can conduct online surveys or experiments to study organisations and individual behaviour.