At present, electron ionization is mainly applied to analyze organic compounds by gas chromatography/mass spectrometry method. Energetic electrons (usually having kinetic energy of about 70 eV) interact with molecules in vacuum (10–5 – 10–4 Pa) yielding typically radical-cations with excess of internal energy. It mostly results in fragmentation of molecular ions which is characteristic for this hard ionization method. Electrons are emitted from a resistively heated metal filament and enter an ionization chamber with neutral gaseous sample. Analytes have to be sufficiently volatile and thermostable. Higher abundance of molecular ions can be achieved by cold electron ionization due to vibrational-rotational cooling of sample molecules expanding into vacuum. Nevertheless, chemical ionization is more often used as a complementary ionization method to determine molecular mass for the case when molecular ions produced by electron ionization are unstable. Both electron and chemical ionization are suitable for coupling of gas chromatography to mass spectrometry. Besides a gas chromatograph, samples can be delivered to the ion source using a reservoir or direct probes. The probes enable the samples to evaporate close to the ionization chamber. Electron ionization represents a simple way of conversion of neutrals to ions and gives reproducible mass spectra well comparable with those in libraries. It definitely contributes to its popularity.