  At sufficiently high baryon density, a quark matter is expected to become a
color superconductor because of the pairing forces mediated by gluons. The
theoretical aspect of this novel phase of the strong interaction is reviewed
with the emphasis on the perturbation theory of QCD at high chemical potential
and low temperature. The derivation of the scaling formula of the
superconducting transition temperature at weak coupling is explained in detail.
The Ginzburg-Landau theory of the color superconductivity is also discussed.
