The purpose of this review is to highlight new ligands that have been immobilized on polymers to form polymer-supported reagents which are selective for the complexation of targeted metal ions from multicomponent solutions. Polystyrene remains the most popular organic polymer support, and its synthesis by suspension polymerization yields beads with a diameter appropriate for large-scale applications. Immobilized ligands include amines, pyridines, imidazoles, oximes, hydroxylamines, Schiff bases, thiols, crown ethers, and a varity of phosphorus ligands. The concepts inherent to reactive ion exchange, as first described by Helfferich, and the recently synthesized dual mechanism bifunctional polymers can be used to build a unified theory of selectivity with polymer-supported reagents. The goal remains to engineer selectivity into polymeric reagents in an a priori manner by understanding the basic principles through which a polymeric reagent can recognize a targeted substrate.