We discuss a recently developed microscopic model for ion conduction in crystals of the beta- and beta "-alumina type, where the charge transport is associated with the motion of certain mobile defects. Monte Carlo simulations of the model provide a good quantitative description of the mixed alkali effect based on the fact that different types of mobile ions have a different preference to become part of a mobile defect. By taking into account the Coulomb interaction between the effectively charged defects and the disorder present in the conduction plane, also the conductivity spectra can be understood. We further show that our model reproduces the non-Arrhenius behavior of the dc-conductivity found in Na-beta "-alumina.