Our everyday lives seem to be controlled by a fundamental asymmetry of nature: The past and the future are not interchangeable, youth will not return, and the dead will not come back to life. Life appears to run preferentially in one direction. Humans have been preoccupied with the meaning of this "time arrow," which appears to determine their fates, ever since the beginnings of philosophy and religion. Since the end of the nineteenth century, this problem has also been tackled with mathematical and physical precision within the framework of thermodynamics. Entropy may be interpreted statistically as a measure of disorder in systems such as a collection of gas molecules in an insulated container. The spontaneous increase of disorder, as well as the decay of ordered systems, is predicted by the second law of thermodynamics. But how are we to understand cosmic evolution, which appears to develop ever more complex ordered systems from simpler ones?