*Immoderate Greatness* explains how a civilization’s very magnitude conspires against it to cause downfall. Civilizations are hard-wired for self-destruction. They travel an arc from initial success to terminal decay and ultimate collapse due to intrinsic, inescapable biophysical limits combined with an inexorable trend toward moral decay and practical failure. Because our own civilization is global, its collapse will also be global, as well as uniquely devastating owing to the immensity of its population, complexity, and consumption. To avoid the common fate of all past civilizations will require a radical change in our ethos—to wit, the deliberate renunciation of greatness—lest we precipitate a dark age in which the arts and adornments of civilization are partially or completely lost.