Duhigg introduces Eugene Pauly, a man with severe amnesia who could not form new conscious memories but could still form new habits. Pauly's case demonstrated that habits are stored in a different brain region (the basal ganglia) than conscious memories (the hippocampus), proving that habitual behavior operates below the level of awareness. Duhigg then explains the habit loop — cue, routine, reward — using MIT laboratory experiments with rats navigating mazes, and introduces the concept of 'chunking': the brain's process of converting a complex sequence of actions into an automatic routine.

Duhigg explains that habits persist because they create neurological cravings. Using case studies from Procter & Gamble's marketing of Febreze and Claude Hopkins's popularization of Pepsodent toothpaste, Duhigg demonstrates that successful habit creation requires not just a cue and routine but a craving — an anticipatory desire for the reward that drives the routine. Cravings are what power the habit loop: without the craving, the cue doesn't trigger the routine.

Duhigg presents the 'Golden Rule of Habit Change': you can't extinguish a habit, but you can change it by keeping the old cue and the old reward while inserting a new routine. He uses the case of Tony Dungy coaching the Tampa Bay Buccaneers and later the Indianapolis Colts, where Dungy's system focused on making players' responses to game situations automatic — replacing slow, deliberate decision-making with habitual, fast reactions. Duhigg also examines Alcoholics Anonymous through the lens of habit science, arguing that AA works because it helps people identify the cues and rewards driving their drinking and substitute new routines (meeting attendance, sponsor calls, fellowship).

Duhigg introduces the concept of 'keystone habits' — habits that, when established, trigger cascading changes in other behaviors. He tells the story of Paul O'Neill's transformation of Alcoa through an exclusive focus on worker safety. By making safety the company's paramount habit, O'Neill inadvertently improved communication, efficiency, quality, and profitability — because the systems needed to track safety also exposed waste, inefficiency, and poor management. Duhigg also discusses how exercise functions as a keystone habit for individuals.

Duhigg examines willpower as a habit that can be strengthened through practice, using Starbucks's employee training program as his primary case study. Starbucks discovered that its most valuable employee skill was not coffee-making but self-discipline — the ability to remain pleasant and professional during stressful customer interactions. The company's LATTE method (Listen, Acknowledge, Take action, Thank, Explain) gives employees a pre-planned routine for handling stress, transforming willpower from a depletable resource into an automatic habit.

Duhigg examines how social movements spread and sustain themselves through habit formation, using the Montgomery bus boycott and the civil rights movement as his primary case study. He argues that social movements succeed through three phases: they begin through the habits of friendship (strong ties), they grow through the habits of community (weak ties), and they endure through new habits of identity that give participants a fresh sense of self. He also examines Rick Warren's Saddleback Church as a case study in building social habits at scale.