Rechargeable solid-state batteries (SSBs) continue to gain prominence due to their increased safety. However, a number of outstanding challenges still prevent their adoption in mainstream technology. This study reveals one of the origins of electronic conductivity, σe, in solid electrolytes (SEs), which is deemed responsible for SSB degradation, as well as more drastic short-circuit and failure mechanisms. Using first-principles defect calculations and physics-based models, we predict σe in three topical SEs: Li6PS5Cl and Li6PS5I argyrodites and Na3PS4 for post-Li batteries. We treat SEs as materials with finite band gaps and apply the defect theory of semiconductors to calculate the native defect concentrations and associated electronic conductivities. Li6PS5Cl, Li6PS5I, and Na3PS4 were synthesized and characterized with UV-vis spectroscopy, which validates our computational approach confirming the occurrence of defects within the band gap of these SEs. The quantitative agreement of the predicted σe in these SEs and those measured experimentally strongly suggests that doping by native defects is a major source of electronic conductivity in SEs even without considering purposefully introduced dopants and/or grain boundaries. We find that Li6PS5Cl and Li6PS5I are n-type (electrons are the majority carriers), while Na3PS4 is p-type (holes). We suggest general defect engineering strategies pertaining to synthesis protocols to reduce σe in SEs and thereby curtailing the degradation mechanism. The methodology presented here can be extended to estimate σe in solid-electrolyte interphases. Our methodology also provides a quantitative measure of the native defects in SEs at different synthesis conditions, which is paramount to understand the effects of defects on the ionic conductivity.