Ionic liquids (ILs) have revolutionized modern synthesis as they are efficient, environmentally friendly media that may be used for a wide range of organic, catalytic, and enzymatic transformations. This chapter discusses their role in catalyzing various reactions like hydrogenation, carbonylation, and hydroformylation, in which the tunable polarity and solvation abilities accelerate rates, selectivities, and stability of the catalysts. In organic synthesis, ILs have been shown to promote important processes, such as nucleophilic and electrophilic substitutions, carbon–carbon coupling, cycloadditions, and multi-component reactions, with better yields and under greener conditions. Stabilizing enzymes, increasing efficiency, and facilitating immobilization for green synthesis are essential to biocatalysis. Their broad electrochemical window, high conductivity, and thermal stability further facilitate photochemical and electrochemical processes.