Permittivity is a very important quantity often used to quantify the behavior of a system under a given electric field distribution in space. The dielectric polarization and associated losses are two main associated effects that are connected through the complex permittivity via its real and imaginary parts respectively. Therefore, the quantitative measure of permittivity is essential for predicting the response of a system, e.g. charge storage characteristics of capacitor/supercapacitor, flux through a charge carrying closed surface, exciton binding energy in soar cells/LEDs, properties of liquid crystals and LCDs, relaxation of dielectrics, microwave absorption effectiveness of shielding materials, response of antenna systems, governing phase velocity of electromagnetic radiation through given medium, designing of metamaterials etc. Permittivity has special relevance in energy storage, energy harvesting, energy conversion, energy transmission, energy dissipation, and even stealth technology; therefore, its thorough understanding is required especially in the context of practical applications. This edited book is the first of its kind that provides a single-source solution to specifically address the fundamentals and applications of permittivity through a comprehensive literature account.