Glass-ceramic and glass-matrix composites are often used as sealants for high temperature applications such as solid oxide cells or gas separation membranes. In addition to thermal stability and electrical insulation, the joining materials provide excellent gas tightness and long-term stability after a viscous sintering process. The composition of the sealant must match the thermal expansion of the joining partners. And the interfacial interactions must be controlled for each specific application to ensure component reliability. Joining tests under realistic conditions, followed by microstructural characterisation using various microscopic techniques, allow the development of materials suitable for their specific purpose. In addition to the material development of the sealant, its application technology is an important aspect for the successful joining of larger assemblies. The near-net-shape production of sealant green sheets allows rapid assembly of system components, but requires the paste to be customised for each composition.