Data is transforming our lives, our work, and the way we think. Through phones, screens, and connected objects, we are creating billions of bytes of data every day. To understand the complexity of this revolution, we travel to four major cities in the world—San Francisco, New York, London, and Paris—to find out how companies like Rocket Fuel, which specializes in data analysis, are changing marketing and online advertising and transforming visitors into consumers. The Economist’s data editor, Kenneth Cukier, and French data specialist Gilles Babinet fully agree: the data war has already begun. All companies are part of it, and one of their primary missions is to reassure consumers on the safety and protection of their personal data.