Ionic liquids (ILs) are composed of various cations and anions. They can be used in many applications in many science and technology disciplines since they exhibit unique properties. They hold promise as engineered materials in many fields, including green solvents/catalysts for chemical reactions, separation sciences, biocatalysts, biopolymers processing, active pharmaceutical ingredients, drug delivery, electrolytes for batteries and supercapacitors, and even for solid-state batteries. Additionally, they can be used as additives in solar cells, including perovskite solar cells, enhancing power conversion efficiency and stability. Recent developments in different aspects of ILs, including physical properties, molecular dynamic simulations, ionic conductivities, active pharmaceutical ingredients, and lubricants, are discussed in this book.