Computer simulation techniques have been used to investigate the defect and transport properties of the zirconia (ZrO₂) oxygen-ion conductor. First, a wide variety of low-valent metal ions are substituted into zirconia, and the energetics of solution investigated. Favourable dopants (on energetic grounds) have been calculated and include CaO, Y₂O₃ , Gd₂O₃ ,and Sc₂O₃ in agreement with observation. Dopant-vacancy clusters are also examined with the results revealing trends with dopant ion size and significant local atomic relaxation. These simulations are extended to encompass the topical area of Nb/Y co-doping in zirconia. Oxygen ion diffusion in yttria-stabilised zirconia is studied by application of molecular dynamics (MD) techniques; our results support models in which diffusion is mediated by oxygen vacancies, with calculated diffusion coefficients and activation energies in accord with tracer diffusion studies.