Kahneman introduces System 1 (fast, automatic, intuitive) and System 2 (slow, deliberate, effortful) as the two protagonists of the book. He demonstrates their characteristics through experiments: the Muller-Lyer illusion (System 1 sees lines of different lengths even when System 2 knows they're equal), the 'bat and ball' problem (System 1 generates an intuitive but wrong answer that most people accept without System 2 checking), and attention experiments showing that System 2 has limited capacity and is easily depleted.

Kahneman catalogs the primary heuristics (mental shortcuts) that System 1 uses and the biases they produce. The anchoring effect: initial numbers influence subsequent estimates even when the numbers are arbitrary. The availability heuristic: events that are easy to recall (vivid, recent, emotionally charged) are judged as more frequent than they are. Representativeness: people judge probability based on similarity to stereotypes rather than base rates. The conjunction fallacy: people rate specific scenarios as more probable than general ones when the specific scenario fits a stereotype.

Kahneman addresses the pervasive human tendency toward overconfidence — in predictions, in judgments of uncertainty, and in assessments of one's own knowledge. He discusses the planning fallacy (systematic underestimation of time and cost for projects), the illusion of validity (unwarranted confidence in one's ability to predict outcomes), and the failure of expert judgment in many domains. He introduces the distinction between 'narrow framing' (evaluating decisions in isolation) and 'broad framing' (evaluating decisions as part of a larger portfolio).

Kahneman presents Prospect Theory, the work for which he received the Nobel Prize in Economics. The theory demonstrates that people evaluate outcomes relative to a reference point rather than in absolute terms; that losses loom larger than equivalent gains (loss aversion); that people are risk-averse in the domain of gains but risk-seeking in the domain of losses; and that people overweight small probabilities and underweight large ones. These findings contradicted the rational-agent model (Expected Utility Theory) that dominated economics for decades.

Kahneman distinguishes between the 'experiencing self' (who lives through events moment by moment) and the 'remembering self' (who evaluates experiences after they end). He demonstrates that these two selves evaluate the same experiences differently: the remembering self is dominated by the peak intensity and the ending of an experience, largely ignoring duration. This 'peak-end rule' means that a painful medical procedure followed by a brief period of mild discomfort is remembered as less painful than the same procedure without the ending — even though the total pain experienced was greater.