Cass Sunstein, the legal scholar best known for developing nudge theory with Richard Thaler, turns his attention to the boundary between acceptable persuasion and unacceptable manipulation. The book proposes a rigorous definition of manipulation for an era when digital technologies give companies and political actors unprecedented power to shape behavior. Sunstein argues that manipulation occurs when someone does not sufficiently engage with a person's capacity for reflective judgment, instead exploiting psychological weaknesses, cognitive biases, or emotional vulnerabilities to steer decisions. This definition matters because manipulation can occur even when the target believes they are choosing freely. The book examines how algorithmic systems create the illusion of choice while systematically pushing users toward predetermined outcomes, from personalized pricing to political microtargeting. Sunstein distinguishes manipulation from coercion, which removes choice entirely, and from legitimate persuasion, which appeals to reason and evidence. He explores cases where the line blurs: is a carefully designed default option a helpful nudge or a manipulative trick? Does it matter whether the manipulator intends to benefit or harm the target? The book addresses manipulation in advertising, social media design, political campaigns, and interpersonal relationships, drawing on philosophy, psychology, and law. Sunstein argues that a clear understanding of manipulation is essential for regulating new technologies and protecting democratic self-governance. He proposes legal and institutional responses while acknowledging the genuine difficulty of drawing bright lines between influence that respects autonomy and influence that subverts it.