A sheet of cellulose paper serving as the stationary support, with a liquid mobile phase advancing by capillary flow, constitutes paper chromatography. Solutes deposited at the origin partition between the moving solvent and water held in the cellulose fibers, so that each species acquires a characteristic retardation factor Rf equal to the ratio of solute travel to solvent-front travel ^[1]. The technique is a planar form of partition chromatography and is therefore distinct from column liquid chromatography, from thin-layer chromatography on a coated plate, and from electrophoresis on paper, which uses an electric field rather than solvent flow as the transport force ^[2]. Essential features are a defined paper grade and humidity, a saturated developing chamber, a specified solvent system, and a visualization method that locates the zones after development. Solvent systems for steroids, purines and pyrimidines, amino sugars, and phenolic acids show that paper-chromatographic selectivity is governed by solute partitioning and solvent composition ^[3]. For corticosteroids, changing the solvent pair changes partition while retaining the same cellulose-supported separation principle ^[4]. Conceptual boundaries exclude quantitative column methods that do not use a paper bed as the separating medium.