The phenomenal growth in chromatography is largely due to the introduction of the versatile technique called high-pressure liquidchromatography, which is frequently called high-performance liquid chromatography [1]. Both terms can be abbreviated as HPLC; see Section 15.1.1 for discussion as to which term is more appropriate. The technique offers major improvements over the classical column chromatography and some significantadvantages over more recent techniques, such as supercritical fluid chromatography (SFC), capillary electrophoresis (CE), and capillary electrochromatography discussed in Chapters 16–17. The number of HPLC applications has increased enormously because a variety of complex samples have to be analyzed to solve numerous problems of scientific interest. Additionally, this demand is being continuously driven by the perpetual need to improve the speed of analysis. The term liquid chromatography (LC) is applied to any chromatographic procedure in which the moving phase is liquid, as opposed to gas chromatography (GC) where a gas is utilized as a mobile phase (see discussion in Chapter 14). Classical column chromatography (see Section 15.1), paper chromatography—a forerunner of thin-layer chromatography (see Chapter 13), and HPLC are all examples of LC. This should clarifywhy it is inappropriate to further abbreviate HPLC to LC; unfortunately,it is still commonly done.دThe major difference between HPLC and the old column chromatographic procedures is the pressure applied to drive the mobile phase through the column. This requires a number of improvements in the equipment, materials used for separation, and the application of the theory. HPLC offers major advantages in convenience, accuracy, speed and the ability to carry out difficult separations. Comparisons to and relative advantages of HPLC over classical LC and GC are given below (see Sections 15.1 and 15.2) to provide the reader a better appreciation of this technique. The readers are encouraged to look up an excellent classical text on HPLC by Snyder and Kirkland [2]. A short review of this technique is provided below; for more detailed discussion see references [1–9].