Over the last three years, there has been an increase in the range of ionic liquids utilised and significant advances in our understanding of the fundamental aspects of these materials. Therefore, the time is apt for Faraday Discussions to provide a foundation for future fundamental challenges and theories that need to be developed to move the subject area forward. This meeting follows on from Faraday Discussion 154 held in 2011 and discusses a range of topics, such as ionicity, structure, electrochemistry, phase behaviour, and introduce areas which were only emerging at that point, such as interactions with liquid and solid interfaces. Ionic liquids have been the focus of intense research over the last 20 years because of their remarkable potential for applications coupled to favourable environmental properties. Ionic liquids are also a medium whereby the nature of the complex interactions (Coulombic, van der Waals and hydrogen bonding) provides a liquid whose structure and properties can be tuned by the choice of the cation and anion wherein reactions are likely to be distinct in terms of activity and selectivity profiles compared with common molecular solvents.