As the current density on a solid oxide fuel cell element increases, the voltage is reduced from that, close to the Nernst potential, obtained under open-circuit conditions due to loss mechanisms, the ohmic resistance of the materials and polarisation effects at the inter-faces, as well as any effect of fuel depletion. The ohmic losses are determined by the materials selected and the dimensions of the component, particularly the electrolyte thickness. Minimisation of polarisation effects requires detailed interface structural control, and advances in this aspect of SOFC engineering are reviewed. Additionally note is taken of variation in performance of SOFC devices consequent on their operational history. (C) 2000 Elsevier Science B.V. All rights reserved.