Abstract: The importance of non-covalent interactions have been demonstrated in many cases. Significant examples within the scope of biomolecular systems are widely known, such as ligand-receptor interactions, protein folding or molecular recognition. Among non-covalent interactions, those between cations and aromatic units are commonly observed between aromatic side chains such as those of Phe, Tyr, His or Trp and amino acids with side chains susceptible of protonation as Arg or Lys. In this work, a computational study has been carried out in systems formed by indole and imidazolium cation, in an attempt to model the interactions between the side chains of Trp and protonated His. Specific solvent effects have been considered by including up to three water molecules in the imidazolium···indole complex. The target of this study is to get insight on the geometric and energetic characteristics of this cation···π interaction, as well as quantifying the effect produced by a small number of water molecules in this system.