This book presents the fundamentals of the separation mechanism of the environmentally benign polymer-based aqueous two-phase systems (ATPS) and applications of ATPS to separation and/or enrichment of inorganic compounds with batch extraction and chromatographic techniques. It first describes the structures of the aqueous polymer phases formed in the ATPS. Then the effect of the background electrolyte on the distribution of ionic solutes is discussed. A subsequent chapter shows the applications of the ATPS batch extraction to analyses of trace metal ions in environmental samples. In the following chapter, the author introduces multistep pH-peak-focusing countercurrent chromatography developed for the separation and enrichment of metal ions with a PEG/Na2SO4 ATPS. Lastly, the author explains that liquid chromatography (LC) with hydrophilic polymer gel columns using an aqueous solution as the mobile phase is regarded as an efficient separation and enrichment technique based on ATPS, and describes multi-step pH peak focusing LC and partition/ion-exclusion chromatographic ion stacking (PIEC ion stacking), newly developed ATPS chromatographic techniques for separation and enrichment of metal ions and inorganic anions in detail. This book helps researchers and engineers design and implement appropriate environmentally benign analytical methods for environmental analyses, reagent manufacturers, metal refining industry, and so on.