Factors controlling electronic conduction in oxides are first discussed. Criteria for achieving predominant ionic conduction are then developed. The role of nonstoichiometry in influencing mixed ionic-electronic conductivity, MIEC, is then evaluated, followed by an examination of the impact of MIEC on electrochemical cell performance, oxygen permeation, and redox kinetics. Means for manipulation and control of MIEC are illustrated with examples drawn from the author’s research on pyrochlore oxides.