Retention governed by a specified class of noncovalent contacts between the solute and a complementary stationary phase, rather than by size exclusion alone, is the conceptual core of interaction chromatography. In analytical practice the term groups hydrophilic-interaction, electrostatic, and related modes in which polar or charged solutes partition into or bind to a water-rich or ionizable layer on the packing ^[1]. Hydrophilic interaction chromatography is the most fully articulated member of this family: a polar bonded phase operated with an organic-rich, water-containing eluent retains polar peptides, nucleic acids, and metabolites in order of increasing hydrophilicity ^[2]. Electrostatic repulsion can be superimposed on that hydrophilic partition so that charged solutes elute isocratically while still experiencing the interaction layer ^[3]. Conceptual boundaries exclude gel permeation and other purely steric exclusions, and they exclude biospecific affinity chromatography, whose ligand is a molecular recognition partner rather than a generic interaction surface. Essential features are a defined interaction chemistry, a mobile phase that modulates that interaction, and a retention order that reports the interaction rather than hydrodynamic volume ^[4].