A glass-ceramic from the MgO-B2O3-SiO2 system, which is a possible candidate for applications in LTCC technology, is presented and described in this monograph. In the introductory part of the book there is a brief description of the basic chemistry and the structure of glasses and the thermodynamics of the crystallization processes of glasses. LTCC technology, the material requirements for substrate applications and the three significant parameters that are used to characterize the microwave dielectric properties are briefly described. Furthermore, a short literature survey of the binary MgO?B2O3 and ternary MgO?B2O3?SiO2 system were also presented. In the first part of the results and discussions is devoted to the synthesis and microwave dielectric properties of Mg2B2O5 and Mg3B2O6 ceramics, as well as Mg3B2O6-based ceramics exhibiting extremely high quality factors up to 240,000 GHz. The second part is focused on the nucleation and crystallization processes of MgO-B2O3-SiO2 glass with the emphasis on crystallization kinetics and the influence of the concentration of a nucleating agent. Finally, the microwave dielectric properties of the obtained glass-ceramics were evaluated.