Ionic liquids (ILs) are a class of low melting point, ionic compounds which have a variety of properties allowing many of them to be sustainable green solvents. These non-molecular solvents possess high thermal stabilities and negligible vapour pressures making them attractive alternatives to environmentally unfriendly solvents that produce volatile organic compounds (VOCs). In this book, the authors present research on the properties, applications and hazards of ionic liquids. Some of the topics discussed include challenges and perspectives of ionic liquids vs. traditional solvents for cellulose processing; ionic liquids as sustainable extractants in petrochemical processing; brönsted acid-base ionic liquids and membranes as ion conducting materials and physical and chemical properties of ionic liquids.