Pre-colum derivatisation in high-perfomance liquid chromatography is mainly used to increase the detectability of compounds requiring analysis. this normally involves reaction with chromogenic or fluorogenic reagents, and tus the use of the more commonly available HPLC detectors. Derivatisation is more rarely used to alter chromatographic properties of compounds or to allow the resolution of optical isomers. Applications of pre-column derivatisation are discussed by reference to the functional group of the substrate involved in the derivatisation reaction.