This topic has been selected from the book, "Analytical Chemistry", written by G. D. Christian and others. Topic - Chromatography: Principles and theory. Learning points: Countercurrent extraction. How chemicals are separated on a column. Types of chromatography- adsorption, partition, ion exchange, size exclusion. Chromatographic nomenclature. Theory of column efficiency. Plate number. Van Deemter equation for packed GC columns. Golay equation for open tubular GC columns. Huber and Knox equations for HPLC. Retention factor. Chromatographic resolution. Separation factor. Chromatography simulation software and databases.