Clear opens with his personal story of injury and recovery, then introduces the book's central concepts: the aggregation of marginal gains, the compound effect of small habits, and the distinction between goals and systems. He argues that outcomes are lagging indicators of habits, and that the most effective path to change is to focus on systems rather than goals.

Clear introduces the three layers of behavior change — outcomes, processes, and identity — and argues that the most powerful and durable changes happen at the identity level. He presents a two-step process for building identity-based habits: decide the type of person you want to be, and prove it to yourself with small wins.

Clear presents the first two laws of behavior change. The 1st Law (Make It Obvious) addresses the cue stage of the habit loop, introducing techniques like implementation intentions, habit stacking, and environment design. The 2nd Law (Make It Attractive) addresses the craving stage, exploring temptation bundling, social norms, and the role of dopamine in habit formation.

Clear presents the final two laws. The 3rd Law (Make It Easy) addresses the response stage, introducing the Two-Minute Rule, friction reduction, and the distinction between being in motion and taking action. The 4th Law (Make It Satisfying) addresses the reward stage, discussing habit tracking, the cardinal rule of behavior change (what is rewarded is repeated), and the role of accountability partners.

The final section addresses advanced topics: the role of genetics and talent in habit formation, the Goldilocks Rule for maintaining motivation, the downside of habits (when mastery leads to complacency), and the concept of deliberate practice within habits. Clear argues that habits are necessary but insufficient for mastery, and that the highest levels of performance require continuous reflection and revision.