Chromatographic separation techniques in combination with selective and sensitive detection systems have gained an enormous popularity in all fields of analytical chemistry. However, even with this variety of available separation-detection systems it is not always possible to achieve the desired detection limit with the necessary precision without, any analyte manipulation or derivatization, When the added reagent is light, the procedure is called photochemical reaction or photoderivatization, As for any derivatization reaction, it can be performed off-line or on-line with the latter being the preferred mode. Any common detector in chromatography can be used to show up the derivatization products, Both custom-built and commercial photoreactors are being used currently, enabling a great variety of derivatization schemes, each one suitable For a particular sample type. Several applications in HPLC and FIA are shown here, involving the analysis of different groups of compounds, as well as some new developments and current trends in the field, (C) 1999 Elsevier Science B.V. All rights reserved.