Leonardo of Pisa-better known today as Fibonacci-was the first Westerner to recognize the power of the Hindu-Arabic number system (featuring the numerals 0 through 9), which offered a much simpler method of calculation than the finger reckoning and cumbersome Roman numerals used at the time. His book Liberabbaci (The book of Calculation) remade the West as the dominant force in science, technology, and large-scale international commerce. Leonardo of Pisa is best known today for discovering the Fibonacci sequence of numbers appearing in biological structures throughout nature, but despite the ubiquity of his discoveries, he has largely slipped from the pages of history. Keith Devlin re-creates the life and enduring legacy of this brilliant yet overlooked mathematician.