Solid Oxide Fuel Cells (SOFCs) with electrical efficiencies above 50 % are considered as very promising option for future decentralized energy conversion. At the Forschungszentrum Juelich (FZJ) planar SOFC stacks are currently being developed and tested at 800°C and up to 10000 h using H2 and methane as fuel. Stacks in the kW class routinely reach power densities of 700 W/cm². Typically the layered material composite of the FZJ-stack consists of cells with yttria stabilized zirconia (YSZ) electrolyte, Ni-YSZ anode and a cathode of lanthanum strontium manganite. The cells are mounted between ferritic steel interconnects. The fuel and air compartment are sealed by glass-ceramics and more recently also by metal brazes. Significant progress in reliable stack operation has been achieved over the past decade. However, problems with thermo-chemical and thermo-mechanical compatibility still remain a major challenge. To illustrate the complexity of material interactions in SOFCs, selected problems related to mechanical failure processes are presented. The role of residual stresses is addressed and fracture phenomena of cell and sealant are described in greater detail.