Including an explicit treatment of electronic polarizability is expected to improve accuracy, especially in molecular systems with large fields due to ions, and in settings where polar molecules move between media of high dielectric constant (e.g., water) and low dielectric constant (e.g., a cell membrane), because changes in dielectric constant lead to changes in the self-polarization of polar molecules. Indeed, the electric field at an atom changes constantly during a simulation because of the movements of other atoms with their partial charges, and these changes induce continual changes in the charge distribution within a molecule and hence of its interatomic forces. These fast charge rearrangements are responsible for the fact that nonpolar organic liquids have static dielectric constants of about 2 and that all organic liquids have high-frequency dielectric constants of about 2. 18-20 Polar liquids have higher static dielectric constants because they have permanent dipole moments which confer orientational polarizability, but this responds more slowly than electronic polarization, so the dielectric constant of a polar liquid falls off with increasing frequency of a time-varying external electric field. Electronic polarization also accounts for the fact that polar molecules become less polar when moved from a high-dielectric constant medium to a low-dielectric medium (or vacuum), where the reaction field in the molecule is weaker. A fixed charge force field cannot account for this change, and is thus expected to lose accuracy when used to model processes where the dielectric environment of a molecule changes much. This may also help explain why water molecules treated as polarizable have a greater tendency to occupy nonpolar binding sites than water molecules treated with fixed charges. We therefore expect that integrating electronic polarizability into a well-parameterized force field can lead to improved accuracy. However, the success of such an advance hinges not only on accuracy but also on how easily the force field can be applied to new molecules and on how well it maintains the efficiency of MD simulations. Pioneering work along these lines dates back to at least the 1970s, and most current polarizable force fields are based on one of three models: induced dipoles, Drude oscillators, or fluctuating charges.