Ionic liquids are salts with melting points below 100°C. They have previously gained attention as potential designer solvents due to their versatility. Their use as such, however, poses some issues regarding their high viscosity and hygroscopicity. For this reason, they are often used in combinations with other solvents, especially water. This work deals mainly with imidazolium-based ionic liquids and the influence of water on their properties and structure. In general, such ionic liquids exhibit a significant degree of local ordering, as their long alkyl chains readily associate, leading to the presence of bicontinuous nanostructures in the bulk. These structures remain present in binary mixtures with water up to a critical point of its concentration, at which hydrophobic ionic liquids undergo phase separation or gel formation, whereas the order is broken in more hydrophilic ionic liquids. This work also includes descriptions and interpretations of the results of some methods that have been used to characterise residual water in terms of its influence on the structure and properties of ionic liquids, as well as the state of water at its lower concentrations, namely the tendency of its molecules to form clusters or remain disassociated and dispersed throughout the bulk. These studies could potentially provide a deeper insight into the complex interplay between water molecules and ionic liquids.