Fifty years ago Francis Crick and James D. Watson proposed the double helix model for the DNA molecule. They believed they had, as Crick put it, discovered the "secret of life," and many agreed. But in the intervening years, science has marched--sometimes leaped--forward, and now the question "What is life?" must be posed once again. In this accessible and fascinating book, Michel Morange draws on recent advances in molecular genetics, evolutionary biology, astrobiology, and other disciplines to find today's answers to the question of life. He begins by discussing the various answers that have been formulated in the past, setting contemporary definitions of life within a rich philosophical and scientific tradition that reaches back to ancient Greece. Then, with impeccable logic and a wealth of appropriate detail, Morange proceeds to lay out the fundamental characteristics that define life. The road to an understanding of life remains incompletely charted, he concludes, but the nature of its final destination is no longer an enigma.