Hailed as one of the greatest mathematical results of the 20th century, the recent proof of Fermat's Last Theorem by Andrew Wiles brought to public attention the enigmatic problem-solver Pierre de Fermat, who centuries ago stated his famous conjecture in a margin of a book, writing that he did not have enough room to show his "truly marvellous demonstration." Along with formulating this proposition - x"y"=z" has no rational solution for n>2 - Fermat, an inventor of analytic geometry, also laid the foundations of differential and integral calculus; established, together with Pascal, the conceptual guidelines of the theory of probability, and created modern number theory. In this investigation of Fermat's life and work, Michael Sean Mahoney provides rare insight into the mathematical genius of a hobbyist who never sought to publish his work, yet who ranked with his contemporaries Pascal and Descartes in shaping the course of modern mathematics.