This European Standard provides guidance in the use of in vivo absorption techniques to quantify chlorophyll-a concentration in marine and fresh waters. This European Standard is comprised of the following: - definition of the equipment requirement; - a priori data and mathematical tools; - recommendations for verification of measurement system performance and consideration of factors that can influence measurements; - listing of the procedures to be implemented. The in vivo photometric absorption technique (IVP) is based on: a) the additive nature of absorption of individual constituents within a suspension; b) the use of a priori knowledge about the absorption features of chlorophyll-a in the wavelength area of approximately 675 nm; c) the absence of other components interfering with spectral features in this region; d) the use of a measurement cell of sufficient length and spectrophotometer of sufficient performance to enable the absorption feature of chlorophyll-a to be identified at the concentration levels required; e) the availability of a suitable algorithm to identify and quantify the distinctive chlorophyll-a absorption feature within a spectral absorption measurement. This European Standard has been prepared by CEN/TC 230 "Water Analysis". The responsible German committee is Working Group NA 119-01-03-05-06 AK "Biologisch-ökologische Gewässeruntersuchung" ("Biological-ecological water examination") of NA 119-01-03 AA "Wasseruntersuchung" ("Water Analysis") at the Water Practice Standards Committee (NAW). The revision of the project on the national level has been funded by the Länderfinanzierungsprogramm "Wasser, Boden und Abfall" (federal funding programme "Water, soil and waste").