Ionic liquids pose challenges and provide solutions for electrospray ionisation mass spectrometry (ESI-MS). Being inherently charged, they are themselves easy to characterise in both positive- and negative-ion mode by ESI-MS, but the challenge arises when using the ionic liquid as a solvent - how does one then identify the solute? Fortunately, an excess of one charged species does not preclude the detection of others, and good data on charged analytes dissolved in ionic liquids can be obtained. In return, ionic liquids can also be used to solve problems in ESI-MS, most notably removing the restriction that only polar solvents can support the electrospray ionisation process.