The defect chemistry, doping behavior, and ion migration in olivine-type materials LiMPO4 (M = Mn, Fe, Co, and N) are investigated by atomistic simulation techniques. The most favorable intrinsic defect type is found to be the cation antisite defect, in which Li and M ions exchange positions. Li migration is found to occur preferentially down [010] channels, following a curved trajectory. Defect association or binding energies for pair clusters composed of combinations of lithium vacancies, antisite cations, and small polaron species are investigated. Migration energies for divalent antisite cations on Li sites suggest that such defects would impede Li diffusion in LiMPO4 to varying degrees. Calculation of dopant substitution energies for cations with charges +1 to +5 indicate that supervalent doping (e.g., Ga3+, Ti4+, Nb5+) on either Li or M sites is energetically unfavorable and does not result in a large increase in electronic (small polaron) species.