Dielectric materials with colossal permittivity show promise for the development and miniaturization of high-performance capacitors. CaCu3Ti4O12 (CCTO) improvement for multilayer ceramic capacitors (MLCCs) has been achieved. CCTO shows a large ε′ of ∼104 over a temperature range. This behavior is due to a potential barrier at the grain boundaries (GBs). CCTO ceramics have an electrically heterogeneous microstructure with semi-conducting grains and more insulating GBs, analyzed by an internal barrier layer capacitor (IBLC) structure model. Therefore, the dielectric properties of these materials can be improved by changing the electrical properties of the grains and GBs. In this context, core-shell approaches to control the GBs have been developed. This chapter presents advanced synthesis techniques (by chemistry way but also by cold plasma) to design the dielectric grains of CCTO by shells of different nature, morphology and crystallinity and shows the impact on the macroscopic properties.