Originally published in 1841, Charles Mackay's compendium of historical mass delusions and irrational collective behavior has never gone out of print and continues to be cited by investors and behavioral economists as one of the most insightful studies of crowd psychology ever written. The sections most relevant to financial markets document three of history's most dramatic speculative manias: the Dutch Tulip Mania of the 1630s, when the price of tulip bulbs rose to extraordinary heights before collapsing catastrophically; the South Sea Bubble of 1720, when shares in a British trading company with almost no real business soared on the basis of nothing more than speculative enthusiasm and insider promotion; and the Mississippi Scheme of 1719-1720, when French financier John Law engineered a similarly spectacular boom-and-bust cycle in paper currency and shares. In each case, Mackay documents the social mechanisms by which rational individuals collectively behaved irrationally — the role of initial success in attracting new participants, the social pressure against skepticism, the creation of new financial instruments that obscured the underlying absurdity, and the eventual self-reinforcing collapse. Though Mackay's historical accuracy has been challenged by modern scholars in some details, the essential psychological portrait of speculative fever remains remarkably accurate to every bubble since his time, from the 1920s to the dot-com era to cryptocurrency.