The Complex World, originally published in Volume 1 of Foundational Papers in Complexity Science, presents an entirely new framing of nature, of the human role in the natural and technological worlds, and what it means to prosper on a living planet. We live in a complex world-meaning one that is increasingly connected, evolving, technological, volatile, and potentially poised for catastrophe. And yet we continue to treat the world as if it were simple: linear, unchanging, disconnected, and infinitely exploitable. Complexity science is an approach to understanding and surviving in a complex world. In this concise and comprehensive introduction, Santa Fe Institute President David C. Krakauer traces the roots of complexity science back to the nineteenth-century science of machines-evolved and engineered-into the twentieth-century science of emergent systems. By combining insights from evolution, computation, nonlinear dynamics, and statistical physics, complexity science provides the first scientific framework for understanding the purposeful universe.