The theory and observations relating to ion velocity distributions in the high‐latitudeFregion are reviewed. The review covers three basic aspects, including methods for calculating ion velocity distributions, experimental evidence for the existence of non‐Maxwellian ion velocity distributions, and the ionospheric consequences of non‐Maxwellian distributions. Specific topics covered include closed form solutions to Boltzmann's equation for a simple collision model, series solutions to the Boltzmann equation for more general collision models, retarding potential analyzer measurements, excitation of VLF electrostatic emissions, ion‐neutral reaction rates, and transport