The quantization of singular field theories, in particular, gauge theories, is one of the key problems in quantum field theory. This book - which addresses the reader acquainted with the foundations of quantum field theory - provides a comprehensive analysis of this problem and techniques for its solution. The main topics are canonical quantization and the path integral method. A description of the Lagrangian BRST quantization is included as are original results obtained by the authors: presentation of the physical spectrum of arbitrary gauge theories, quantization of singular theories with higher derivatives and with time-dependent constraints, and the treatment of relativistic point-particles.