Zirconium and hafnium diborides and carbides belong to the class of materials defined UHTCs (Ultra-High-Temperature-Ceramics). These materials are of particular interest because of the excellent and unique combination of properties which make them attractive candidates for high temperature applications where corrosion-wear-oxidation resistance are demanded. So far, the effort of the scientific community has been mainly directed to the study of processing procedures to improve the densification of these compounds. As very refractory compounds, these materials are very difficult to sinter to full density and temperatures as high as 2000-2400 degrees C and pressure-assisted techniques are required to achieve dense materials. The addition of sintering aids has represented a strategy to overcome the intrinsic low sinterability of these highly refractory compounds. However the reaction between sintering additives, the matrix and its oxides (ZrO2, HfO2, B2O3..) during the high temperature stage leads to formation of intergranular phases and/or solid solutions, which, in turn affect the high temperature properties. The control of secondary phases composition is therefore of paramount importance for tailoring the performance of these materials. As there is still lack of knowledge on these microstructural aspects, the main aim of this work was to characterize Ultra-High-Temperature-Ceramics via electron microscopy techniques.