The ionic liquid research area arises at the same time as classical molecular dynamics simulations of large systems and ab initio molecular dynamics simulations of medium sized systems become feasible due to technical progress. Several unique ionic liquid features were uncovered by computational investigations and later confirmed by experimental investigations which illustrates the steady increasing role of theoretical chemistry over the last years. Nonetheless, ionic liquids are a challenge for computational chemistry, because induction as well as dispersion forces play a significant role for interaction energy and equilibrium structure. Force fields and Kohn-Sham density functional methods were developed and validated for ionic liquids which allow reliable multi-scale investigations of ionic liquids today.