Liquid–liquid extraction is one of the most widely used separation methods that exploits the difference in solute partitioning between two immiscible liquid phases and is usually performed using water and water-immiscible organic solvents as its separation mechanism has already been clarified by many studies. On the other hand, aqueous two-phase systems (ATPSs), formed by mixing two water-soluble but mutually incompatible compounds in water, have emerged as an environmentally friendly platform for liquid–liquid extraction. Although ionic liquids (ILs) and deep eutectic solvents (DESs) have been recently introduced as new types of phase-forming components for ATPS, polymer-based ATPS, especially with polyethylene glycol (PEG), is the most widely used platform for liquid–liquid extraction at present in the scope of the green chemistry concept. The author will demonstrate in this book the effectiveness of polymer-based ATPS as chemical separation systems by describing the fundamentals of the separation mechanism of the ATPS and its applications to separation and/or enrichment of inorganic compounds with a conventional batch-wise liquid–liquid extraction technique and newly developed ATPS liquid chromatographic techniques. This chapter provides the fundamentals of polymer-based ATPS and an overview of the contents of this book.