Sintering is a crucial step for structural ceramics as it leads to the elimination of porosity and hence to the achievement of high thermo-mechanical performances. The class of materials known as ultra-high temperature ceramics (UHTCs) requires a combination of severe consolidation process, such high temperature above 2000°C and the application of pressure, which leads to coarse microstructure with poor mechanical behavior. UHTCs include borides and carbides of transition metals and are characterized by melting point above 3000°C and physic-chemical properties suitable for application in the aero-space field. Efforts have been concentrated on the mitigation of the sintering conditions and the addition of secondary phases forming eutectics or transient liquid phase is the most successful approach. Drawbacks of multiphase system can be related to the precipitation of reaction phases at the triple points or formation of grain boundary glass that may hamper the mechanical behavior at high temperature. Transmission electron microscopy is the most suitable method to investigate the microstructure at nano-scale length and obtain a set of information not achievable with other analytical techniques. A series of UHTC systems is presented and the process-microstructure-properties relationships are discussed.