The effectiveness of aliovalent doping to enhance ionic conductivity through increasing the concentrations of charge-carrying mobile defects depends on two conditions. First, aliovalent doping should preferentially increase the concentrations of charge-carrying mobile defects ahead of competing immobile defects. For lithiumion solid electrolytes, doping should preferentially increase the concentration of either lithium vacancies or lithium interstitials. Second, any increase in the concentration of charge-carrying defects should produce a corresponding increase in ionic conductivity. This may not be the case if the ionic conductivity has some dependence on defects other than those being targeted through doping.