There are three primary advantages of continuum solvation models. The first is a reduction in the number of degrees of freedom in a system. While being approximate, these models provide enough influence over solute charge distributions and inter-atomic forces to allow attention to be focused on other aspects, such as a more precise quantitative treatment of the solute structure or dynamics, rather than further refining the description of solvent effects. The second advantage of continuum solvent models lies in their ability to provide an accurate treatment of strong, long-range electrostatic forces that often dominate solvation phenomena. Additionally, most explicit-solvent simulations often neglect solute electronic polarization due to computational costs, while there is over 20 years' worth of experience using continuum theory in handling polarizable solutes. There are numerous applications where simple continuum solvation models are useful. In situations involving large molecules or demanding many evaluations of the solvation energy, alternative higher-level methods become impractical. Continuum solvation models have proven particularly valuable for calculating electrostatic fields and solvation free energies of large molecules, titration curves, solvation effects in quantum mechanical calculations and non-equilibrium solvation energies. Although implicit models based on a dielectric continuum approximation are significantly more efficient, they generally provide less accuracy due to a lack of explicit dynamical information about the solvent degrees of freedom. Continuum models remove the difficulties associated with the statistical sampling of solvent configuration space, but they do so at the cost of losing molecular-level detail. Another limitation in continuum solvent models is their poor performance with reactions in solutions. Unlike the situation in the gas phase, where reactants proceed to products along the pathway in isolation, the presence of solvent molecules in a solution constantly interacts with the reactants. One way in which a solvent medium can impact the reaction rate is by modifying the activation (free) energy. This modification can have a significant influence, as a change of only 2 kcal can alter the room temperature rate by a factor of 30. Good examples here are ionic and polar reactions in polar solvents. Despite the progress in theory and computation of microscopic properties of polar solvent, calculations based on the well-known Transition State Theory (TST) and the continuum dielectric model of the solvent suffer from being unstable and unreliable due to polarization catastrophe.