Solid-Oxide-Fuel-Cell systems are efficient devices to convert the chemical energy stored in fuels into electricity. The functionality of the cell depends among other features on to the structural integrity of the ceramic electrolyte, as one of its main tasks is to separate the fuel- and the airside from one another. In the case of mechanical failure of the electrolyte, leakage will cause performance losses and might even lead to the rapid degradation of the cell and hence of a whole fuel cell stack. One reason for leakage would be fracture. So, in order to evaluate the probability for the electrolyte to fracture, two aspects have to be elucidated. The stress environment the electrolyte is subjected to and its strength and strength distribution for the given system specific environment. This thesis’ aim is to exemplarily describe both aspects for a commercial solid oxide fuel cell being operated within a state of the art fuel cell system, which is covered in four individual and consecutive studies. In a first study, the failure mechanisms and the actual causes for fracture of electrolyte supported SOFCs were investigated, which were run within the current fuel cell system of the Hexis AG/Switzerland under lab conditions or at customer sites for up to 40,000 h. Several operated stacks were demounted for post-mortem inspection, followed by a fractographic evaluation of the failed cells. The respective findings are then set into a larger picture including an analysis of the present stresses acting on the cell like thermal and residual stresses and measurements regarding the temperature dependent electrolyte strength. For all investigated stacks, the mechanical failure of individual cells can be attributed to locally acting bending loads, which rise due to an inhomogeneous and uneven contact between the metallic interconnect and the cell. Two subsequent studies evaluate the strength distribution, fracture toughness and the elastic constants of several zirconia compounds and tapes for the whole relevant temperature range. In addition, their reliability is assessed with respect to temperature and subcritical crack growth (SCCG). It was found that the strength is only influenced by temperature through the change in fracture toughness. SCCG has a large influence on the strength and the lifetime for intermediate temperature, however its impact becomes limited at temperatures higher than 650°C. In this context the tetragonal 3YSZ and 6ScSZ behave quite different than the cubic 10Sc1CeSZ. This cubic compound shows at room temperature a comparably low reliability but becomes competitive to the tetragonal ones at operating temperatures. Within the final study, the electrolyte is considered as part of the actual cell with respect to the influence of temperature and ageing. Ball-on-3-Ball bending strength tests and fractography conducted on anode and cathode half-cells reveal the underlying mechanisms, which lead to cell fracture. They were found to be different for the cathode and the anode side and that they change with temperature and ageing. Both anode and cathode sides exhibit the lowest strength at T = 850°C being in their initial state, which is significantly lowered compared to the strength of the bare electrolyte. This reduction is the consequence of the formation of cracks in the electrode layers, which either directly penetrate into the electrolyte (anode side) or locally increase the stress intensity level of pre-existing flaws of the electrolytes at the interface (cathode side).