Within the frame of the European project `Clean Seas' we have used a multisensor approach for studying the imaging of different oceanic phenomena by satellite-borne sensors working at different (optical, IR, and microwave) frequencies. For this purpose, algae blooms in the Baltic Proper and a river plume in the north-western Mediterranean have been chosen as case studies. The used sensors are the Synthetic Aperture Radar (SAR) and the Along-Track Scanning Radiometer (ATSR) aboard the ERS-2, the Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA satellites, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) on the SeaStar satellite, the Thematic Mapper (TM) onboard Landsat-5, Enhanced Thematic Mapper (ETM) on the Landsat-7 satellite, the Synthetic Aperture Radar (SAR) onboard Radarsat, the Wide Field Scanner (WiFS) onboard the Indian IRS-1C satellite, the Modular Optoelectric Scanner (MOS), and the Linear Imaging Self Scanner (LISS) onboard the IRS satellites. Each data set comprises remote sensing data from the same spot acquired by at least three different sensors within a few hours (i.e. quasisimultaneously) .