We reviewed different theories of the ion–ion recombination coefficient α, which describes the reaction and mutual neutralisation of two oppositely charged ions. We focussed on temperature and pressure dependencies and the atmospheric altitude range between 0–50 km. We compared the theories for atmospheric conditions and identified the most favourable value for standard conditions, 1.7 × 10⁻⁶ cm³ s⁻¹, and recommend different theories or parameterisations depending on the altitude.