Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.

The charge of QCD is called "color" and comes in three fundamental values called "red", "green", and "blue", to which are coupled eight distinct states of a single vector boson called a "gluon".

Unlike the photon in QED, gluons are themselves color charged objects, which means the theory is complicated by the existence of three- and four-gluon vertexes.

A important feature of the strong interaction is confinement, which demands that all all objects able to exist at large distance separation must be color-neutral: either adding equal amounts of red, green, and blue (as in baryons); or by adding color and anti-color (as in mesons).