Solvent extraction provides the basis for important separation and concentration processes that are widely used in hydrometallurgy, waste treatment, ­ e ffluent purification, and material preparation 1– 3 . In a typical extraction process, the complexing agent, the extractant, plays a key role and is responsible for the phase transfer of one or more species of interest between the particular liquid– liquid phases employed. At present, four categories of metal extractants are used in industry: acidic and chelating cation exchanging reagents as well as solvating and anion exchanging reagents. An aim of this review is to show how supramolecular concepts may play a role in influencing extraction processes with the focus placed on the more recent work. Since both calixarene-derived extractants and metal salt extractants are treated in other chapters of this book, these areas have been given less emphasis in the discussion that follows.