This book reviews recent developments in the field of ionic liquids (ILs), emerging as high-tech materials for a host of societally important applications. ILs are composed of ions chosen to achieve maximum destabilization of the crystalline solid state of the substance. In Walden's original paper (1914) which examined the electrical conductivity and the molecular size, of mono-, di- and tri-alkylammonium nitrates, he described this class of substances as "salts that exist in a liquid state under standard conditions due to the peculiarities of their chemical structure" and mentioned that "the behavior of these organic salts at temperatures below or around 100 C resembles experiences with inorganic molten salts." This book provides a review of ILs, their chemistry, and their applications.