  An introduction into the theory of boundary critical phenomena and the
application of the field-theoretical renormalization group method to these is
given. The emphasis is on a discussion of surface critical behavior at bulk
critical points of magnets, binary alloys, and fluids. Yet a multitude of
related phenomena are mentioned. The most important distinct surface
universality classes that may occur for a given universality class of bulk
critical behavior are described, and the respective boundary conditions of the
associated field theories are discussed. The short-distance singularities of
the order-parameter profile in the diverse asymptotic regimes are surveyed.
