Self-organized criticality, the spontaneous development of systems to a critical state, is the first general theory of complex systems with a firm mathematical basis. This theory describes how many seemingly desperate aspects of the world, from stock market crashes to mass extinctions, avalanches to solar flares, all share a set of simple, easily described properties. " . . . a 'must read' . . . Bak writes with such ease and lucidity, and his ideas are so intriguing . . . essential reading for those interested in complex systems . . . it will reward a sufficiently skeptical reader" -NATURE " . . . presents the theory (self-organized criticality) in a form easily absorbed by the non-mathematically inclined reader" -BOSTON BOOK REVIEW "I picture Bak as a kind of scientific musketeer; flamboyant, touchy, full of swagger and ready to join every fray . . . His book is written with panache. The style is brisk, the content stimulating. I recommend it as a bracing experience" -NEW SCIENTIST