A dielectric is a material, which consists entirely of chemically bound chargesChargesbound that lack the freedom to move between molecules. As a consequence, the material is incapable of sustaining a steady current, and functions instead as an insulator. Dielectrics can be classified as polar and non-polar. The molecules in a non-polar dielectric do not have permanent electric dipole moments. In a uniform applied electric field, the electronsElectron and nuclei in the atoms in a non-polar dielectric suffer only displacementsDisplacement from their equilibrium positions. By contrast, in a polar dielectric, the molecules possess permanent electric dipoles. In gaseous and liquid polar dielectrics and some solids, the molecules can rotate under action of the applied field. In both polar and non-polar dielectrics, the applied electric field produces charge distribution distortions, which are known as “polarization.” The combination of the external electric field and the electric field associated with this internal polarizationPolarization serves to define a new vector called the displacement. Lorenz, Debye, and Onsager, as well as many others, have attempted to link the charge distortion in a single molecule to the displacement observed in the macroscopic dielectric. In these models, a given molecule is regarded as carving out a cavityCavity for itself in an otherwise continuous dielectric. The molecule in the cavity interacts with the applied field as well as the polarization raised in the surrounding continuous dielectric, thus sensing a local electric field which exceeds the applied field. These so-called semi-continuum theoriesSemi-continuum theory provide a basis for deriving equations for the density dependence of the electric permittivity, also known as the dielectric constantDielectric constant. When combined with experimental data, the local field concept can be used to determine molecular permanent electric dipole moments, molecular polarizabilities, dipole solvation energies, and the effects of the solvent on the absorption spectrumAbsorption spectrum of a solute.