A review is given of glow discharge techniques which are useful for conditioning vacuum vessels for high-vacuum applications. Substantial development of glow discharge techniques has been done for the purpose of in situ conditioning of the large ultrahigh vacuum systems for particle accelerators and magnetic fusion devices. In these applications the glow discharge treatments remove impurities from vessel surfaces in order to minimize particle-induced desorption coefficients. Cleaning mechanisms involve a mixture of sputtering and ion(or neutral)-induced desorption effects depending on the gas mixture (Ar/O2 vs H2) and excitation method (dc, rf, and electron cyclotron resonance). The author will review the methodology of glow discharge conditioning, diagnostic measurements provided by residual gas and surface composition analysis, and applications to vessel conditioning and materials processing.