To build is to be human. Creating things is part of our nature, and our principal mode of survival. The things we have built throughout history have always relied on design to turn discoveries into workable tools. But the underlying premise of design is that we can reason about the pieces and connections that make a thing work. From the stone axe to the rocket engine, design is predicated on our ability to see causal connections between the parts of a system. But what happens when such causality is no longer apparent? When the things we must build to solve our challenges have inner workings that cannot be discerned? The answer lies in nature. Nature fashions truly complex objects that solve categorically hard problems. Complex things produce their most important outputs via emergence, whereby a system's structures and behaviors arise in ways that cannot be designed. This book argues that we are entering an age where humanity must build truly complex things to continue our progress. This means learning to build as nature builds, and as it turns out, forces us to redefine knowledge and skill, and more broadly our scientific and engineering paradigm.