The theory, central to Taleb's critique of standard statistics, that many real-world distributions have "fat tails"—extreme events are much more likely than the normal distribution predicts. In a normal distribution, extreme events are virtually impossible; in fat-tailed distributions, they happen regularly. Financial markets, pandemics, wars—all are fat-tailed. Fat Tails Theory argues that we have been using the wrong statistical tools, underestimating risk, pretending the world is safer than it is. The theory explains why Black Swans are not as rare as we think: they're rare in thin-tailed distributions, normal in fat-tailed ones. Fat Tails is the mathematics of humility, the quantification of our ignorance, the proof that we live in a world where the improbable happens—and we'd better be ready.