Abstract: Mixed ions and electron conductors (MIECs) are an important family of electrocatalysts for electrochemical devices, such as reversible solid oxide cells, rechargeable metal–air batteries, and oxygen transport membranes. Concurrent ionic and electronic transports in these materials play a key role in electrocatalytic activity. An in-depth fundamental understanding of the transport phenomena is critically needed to develop better MIECs. In this brief review, we introduced generic ionic and electronic transport theory based on irreversible thermodynamics and applied it to practical oxide-based materials with oxygen vacancies and electrons/holes as the predominant defects. Two oxide systems, namely CeO₂-based and LaCrO₃-based materials, are selected as case studies to illustrate the utility of the transport theory in predicting oxygen partial pressure distribution across MIECs, electrochemical electronic/ionic leakage currents, and the effects of external load current on the leakage currents.