Differential adsorption of solutes onto a solid surface from a liquid or gaseous mobile phase is the retention mechanism that defines adsorption chromatography. The stationary phase is an adsorbent—silica, alumina, a thiophilic or hydrophobic solid, or a related high-surface-area material—whose sites compete with the eluent for the analyte ^[1]. Retention is therefore an interfacial phenomenon, not a bulk partition into a liquid film, and it is described by adsorption isotherms and by gradient-elution theory that relates solvent strength to surface coverage ^[2]. Conceptual boundaries exclude partition chromatography, ion-exchange chromatography whose primary interaction is electrostatic, and affinity chromatography that requires a biospecific ligand, although an adsorbent bed may be operated in an expanded-bed geometry without changing the adsorption definition ^[3]. Essential features are a reversible adsorption equilibrium, a mobile phase that can modulate surface activity, and a column or bed in which adsorption and desorption occur repeatedly as the solute band migrates. Linear elution adsorption chromatography treats the low-concentration limit in which the isotherm is linear and retention is independent of load.