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Richard H. Thaler is a leading economist and a bold and original researcher. A professor at the University of Chicago—the center of economic orthodoxy, where Milton Friedman and Eugene Fama spent most of their careers—, he is one of the most relevant critics of the efficient market hypothesis. His works in finance, often of great simplicity, changed the dominant paradigm. While there are few arbitrage opportunities in financial markets, they do exist and can be enduring. Rational investors do not always have the last word and prices often tend to over- or underreact to information. It is always particularly difficult to “beat the market,” even for knowledgeable practitioners, but asset prices do not fully capture the fundamentals, and speculative bubbles are common. In short, the markets are not populated with homo oeconomicus but homo sapiens.