Volume 6 of the "Encyclopedia of Mass Spectrometry" serves as a concise review of the current ionization methods used in mass spectrometry today. The majority of chapters include tutorials describing the principles and instrumentation applied to each method. Techniques covered include ionization of gas-phase molecules by particles and photons, ionization of ion-molecule and molecule-molecule reactions, ionization in strong electric fields, spray methods for ionization, desorption ionization by particle bombardment and biophysical applications of desorption and spray ionization. The selection of appropriate methods of ionization for analysis of various substances is also considered. It considers appropriate methods of ionization for analysis of various substances.