Ionization gauges are commonly used for pressure measurement from the lowest achievable pressures to 10−2Torr. Two types exist: In the hot cathode gauge (HCG) the source of the ionizing electrons is a thermionic cathode. In the cold cathode gauge (CCG) it is a circulating space charge current of electrons trapped in crossed electric and magnetic fields. Indications of both types of gauge are gas dependent. Relative sensitivities for different gases are not the same for different HCGs or CCGs, or for HCGs compared to CCGs. Both types of gauge are susceptible to contamination. The filament heating power of the HCG is sufficient to cause local outgassing of adsorbed material, and possibly large pressure errors. Each has certain spurious effects. The lower limit of the HCG is set by x‐ray photocurrent and electron impact desorption. The CCG is subject to discontinuities in the current versus pressure characteristic. Starting of the discharge in a CCG may be delayed at ultrahigh vacuum (UHV). Pumping speeds of both types of gauge are of magnitude 10−2l/s, so that gauge pumping is usually negligible.