Due to their unique yet tunable properties, "ionic liquids" are widely used as solvent and/or catalysts in many organic reactions. From the past two decades, there are increasing literature reports on the usage of ionic liquids (ILs) even in the polymerization chemistry as solvent, cosolvent, initiator, catalyst, or metalcomplexing ligand. IL-mediated polymerization offers faster rates, higher molecular weight polymers, good yields, easy separation of products, and recovery and reuse of catalyst over conventional organic solvent-mediated polymerization process. In this chapter, we discussed various ILs (literature reported) that were employed as solvents and/or catalysts in different kinds of polymerization reactions and their advantages, influence on polymer properties, recovery, and polymerization kinetics. © Springer-Verlag Berlin Heidelberg 2015.