Quantum chromodynamics, or QCD, is the theory that describes the strong nuclear force. It is a quantum field theory in which quarks interact with one another by exchanging force-carrying particles called "gluons." It has two striking features that distinguish it from the weak and electromagnetic forces. First, the force between two quarks remains constant as the quarks are pulled apart. This explains why single quarks have never been found in nature. Second, quarks and gluons interact very weakly at high energies. QCD is an essential part of the Standard Model and is well tested experimentally; however, calculations in QCD can be very difficult and are often performed using approximations and computer simulations rather than solved directly.