Field ionization and field desorption are described and distinguished. Field ion and field desorption emitters of various types are available, and activation processes for these emitters have been devised. Ion sources and mass spectrometers for field ionization, and the mass spectra produced, are discussed. The investigation of surfaces and surface processes by field ionization mass spectrometry is considered, and the importance of chemical reactions on the emitter surface is emphasized. Field ionization can be used to investigate the kinetics and mechanisms of ionic dissociation reactions. The uses of field ionization mass spectra for analytical and structural investigations are briefly summarized.