We discuss oxygen and cation diffusion processes in oxygen ion conductors. While the high oxygen diffusivity determines the proper oxygen ion conductivity, slow cation diffusion processes are important for sintering and degradation processes. In the first part of the paper we discuss an analytical model for the ionic conductivity of a strongly acceptor doped, fluorite-type oxygen ion conductor, i.e. a concentrated solution of AO2 and BB2O3. The model can be applied, e.g., to yttria doped zirconia (YSZ) and gives a qualitative explanation of the observed maximum of the conductivity as a function of the dopant fraction. The model considers nearest neighbor interactions between oxygen vacancies and dopant cations, which may be negligible, attractive or repulsive, and jump barriers that depend on the nature of the cation-cation edge that has to be crossed during a jump between adjacent oxygen sites. In the second part we discuss cation diffusion processes in doped lanthanum gallates (LSGM). The experimental results of nearly identical cation diffusion coefficients in the A- and B-sublattices of the perovskite LSGM can be explained by a bound defect cluster mechanism containing cation vacancies of both the Aand the B- sublattice and anion vacancies.