The Medium Resolution Imaging Spectrometer (MERIS) will be flown on the Envisat mission in 1999 and will provide the user community with a unique instrument for monitoring important water quality parameters in coastal waters. The instrument will be of special interest for coastal zone research projects such as the International Geosphere Biosphere Program (IGBP), Land Ocean Interactions in the Coastal Zone (LOICZ) and for environmental impact monitoring, assessment and management programmes. MERIS characteristics include nine spectral channels (out of 15) covering the visible spectral range (410-705nm) which are optimized to the radiance level of water surfaces. Within this range there are bands dedicated to mapping concentrations of suspended particulate matter, phytoplankton, gelbstoff (or coloured dissolved organic matter) and bands for determining sunlight-stimulated fluorescence of phytoplankton chlorophyll. Its spatial resolution of 300m and revisit period of 3 days are well suited to the observation of most of the phenomena which are of interest for coastal water quality research and management. New algorithms will have to be developed for computing the different water constituents from the observations and for atmospheric correction of turbid waters. Examples of applications, for which the design of MERIS is optimized, are presented.