This up-to-date review also serves as an introduction to Heavy Quark Effective Theory (HQET) - a new approach to heavy quark physics problems in Quantum Chromodynamics (QCD). Heavy quark physics is one of the most active areas of high-energy physics, expecially at this time when B-factories at SLAC and KEK are starting experiments, and yet other facilities are being developed. HQET has already led to significant progress in our understanding of heavy quark physics during the last 10 years. The book also contains a detailed discussion of the methods of calculation used in HQET, along with numerous illustrations.