Ionic liquids are organic salts with potential for intensification of liquid–liquid processes. The structures of a range of significant ionic liquids are presented. The focus is on intensification of classical organic reactions and separations using ionic liquids in liquid–liquid systems. Their role as reaction media is briefly reviewed. Phase equilibrium properties of several liquid–liquid systems involving ionic liquids are described, demonstrating their potential for azeotrope breakage in vapor–liquid and liquid–liquid systems. Application of ionic liquid technology to phase-transfer catalysis is discussed, with inclusion of the classic example of the dimerization of butene to iso-octenes. The potential role of ionic liquids for the exploitation of biocatalytic processes is highlighted, with discussion of the potentially toxic effects on living biomass and on the activity of enzymes. The significance of the toxicity of some ionic liquids is summarized, together with a short discussion of potentially wider environmental impacts. The degradability of ionic liquids is an important part of environmental assessment that is also considered.