The world you can feel and touch is built of atoms, the smallest identifiable chunks of matter. Yet the heart of each atom is itself a whole new world, a world populated by quarks: indivisible, vanishingly small, the ultimate building blocks of our universe. This inner world where quarks reign is subject to new and unfamiliar rules, the rules of the quantum world. Colossal particle accelerators enable physicists to bring this inner world into focus, and have helped them shape a theory respectful of quantum rules that explains how quarks feel one another's presence. The Quantum Quark is the story of that theory: quantum chromodynamics.