The decisions you make don't respond solely to your will but reflect the influences, subtle or gross, of the social environment in which you live: sometimes to avoid confrontation, other times to avoid making mistakes, often because it strengthens ties with people you care about. Conformity is the more or less automatic, often unconscious, response with which we adapt to the opinions of others, their expectations, and their preferences. In this short essay, Cass R. Sunstein explains how social pressure works, why like-minded groups are prone to polarization, how much reputation weighs on our actions, and how authority figures can trigger "cascades": a series of individual decisions that are reproduced as if responding to an external design. This original approach can be applied to matters as minor as what song to listen to right now, or as matters of great importance as which name to mark on the ballot. With a range of analytical approaches ranging from psychology to law, from economics to communication, and examples encompassing the composition of courts and the dynamics of social networks, the author presents a simple and versatile theory for understanding the potential risks of conformity and the virtues of dissent.