Composite materials are increasingly used for dielectric applications, i.e., applications that make use of electrically insulating or nearly insulating behavior. This is because of the need of the electronic industry for dielectric materials in electrical insulation, encapsulation, substrates, interlayer dielectrics in a multilayer ceramic chip carrier, printed circuit boards, and capacitors, and because of the rising importance of smart structures that use dielectric materials for piezoelectric, ferroelectric, and pyroelectric devices that provide sensing, actuation, etc. The dielectric constant of a material depends upon the polarizability of its molecules and is determined by different contributions: interfacial, dipole, atomic and electronic polarizations. Atomic and electronic polarizations are instantaneous and do not affect the dependency of the dielectric constant on frequency. Dipole polarization is due to the presence of polar groups in the matrix and fibres.