Ionic liquids (ILs) are promising materials for a broad range of applications: electrolytes in batteries and supercapacitors, materials for gas separation and solvents for catalysis. However, realisation of the full potential of ILs is hindered by a lack of understanding in how varying IL composition (the identity of the ions) affects physical properties. Multiple key IL properties are determined by electronic structure, including reactivity, electrochemical window and interactions with light. Therefore understanding IL electronic structure is an essential step towards the rational design of ILs. The ability of small model systems to capture bulk IL electronic structure is investigated by combining ab initio calculations with X-ray spectroscopy (Chapter 4). Single ions surrounded by a (computationally cheap) solvent continuum model are demonstrated to be capable of capturing bulk IL electronic structure. Chapter 4 also contains an investigation into how intermolecular interactions affect the electronic structure of ions in ILs, with particular focus on molecular orbital (MO) energies. Water will be present at significant concentrations in many IL applications, therefore it is important to understand the effects of water on IL properties. The effects of water on IL electronic structure are investigated by combining photoelectron spectroscopy with calculations (Chapter 5). Water significantly stabilises anionic MOs, which is rationalised in terms of dielectric constants. Interactions in IL:water mixtures are investigated using a range of computational methodologies, including natural bond orbital (NBO) and atoms in molecules (AIM) analyses. Charge distribution, often represented by atomic charges, is an important aspect of IL electronic structure. However, different methods for calculating atomic charges lead to qualitatively different charge distributions. Chapter 6 focuses on determining the most suitable charge assignment method by using results from X-ray spectroscopy to validate calculated charges.