Rechargeable solid-state batteries continue to gain prominence due to their increased safety. However, a number of outstanding challenges have prevented their adoption in mainstream technology. In this study, we reveal the origins of electronic conductivity (s_e) in solid electrolytes (SEs), which is deemed responsible for solid-state battery degradation, as well as more drastic short-circuit and failure. Using first-principles defect calculations and physics-based models, we predict s_e 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. Our experimental measurements of the band gap of tetragonal Na₃PS₄ confirm our predictions. The quantitative agreement of the predicted s_e in these three materials and those measured experimentally strongly suggests that self-doping via native defects is the primary source of electronic conductivity in SEs. In particular, we find that Li₆PS₅X are <i>n</i>-type (electrons are majority carriers), while Na₃PS₄ is <i>p</i>-type (holes). Importantly, the predicted values set the lower bound for s_e in SEs. We suggest general defect engineering strategies pertaining to synthesis protocols to reduce s_e in SEs, and thereby, curtailing the degradation of solid-state batteries. The methodology presented here can be extended to investigate s_e in secondary phases that typically form at electrode-electrolyte interfaces, as well as to complex oxide-based SEs.