Polar solutes that partition into a water-enriched layer held on a hydrophilic stationary phase, while the bulk mobile phase is a water-miscible organic solvent rich in acetonitrile or another polar organic, define hydrophilic interaction chromatography (HILIC). Retention increases with solute hydrophilicity and decreases as the water content of the eluent rises, which is the inverse of reversed-phase behavior ^[1]. Stationary phases include underivatized silica, amide, zwitterionic, and other polar bonded phases; the mobile phase is typically an aqueous buffer mixed with a high fraction of organic solvent ^[2]. Electrostatic repulsion hydrophilic interaction chromatography (ERLIC) superimposes Coulombic repulsion on the HILIC partition so that charged solutes can be eluted isocratically ^[3]. Conceptual boundaries exclude normal-phase adsorption on dry silica with nonpolar eluents that lack a water-rich layer, and they exclude ion-exchange chromatography whose retention is purely electrostatic. Essential features are a hydrophilic surface, a water-containing but organic-rich eluent, and a retention order governed by hydrophilic partition, possibly modulated by ionic interactions ^[4].