Ionic liquids challenge conventional descriptions of fluids in a variety of ways. In the past, it has been common to oversimplify their behaviour and to ignore the complexity that was possible, given the burgeoning range of ionic liquids available. Here we discuss what is meant by the term ‘ionic liquid’ and some of the key fundamental issues in understanding their properties. This Research Front collects together a variety of papers that constructively challenge, investigate, and extend our understanding of the field of ionic liquids.