The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) is the successor ocean color imaging system to the Nimbus-7 Coastal Zone Color Scanner (CZCS). The SeaWiFS calibration and validation effort includes spacecraft, atmospheric, sea surface, subsurface (or in situ), plus laboratory and data analysis components which require pre- and postlaunch activities. The most important goals of this effort are to produce water-leaving radiances with an uncertainty of 5% in clear-water regions and chlorophyll a concentrations within ±35% over the range of 0.05-50 mg m -3. The first objective requires field instruments with a calibration and measurement capability on the order of 1%; because these challenging in situ measurements will be acquired from a variety of field instruments over the five-year mission interval, a measurement assurance program is required. This program consists of several activities: an accurate prelaunch characterization and calibration of the SeaWiFS instrument; a Marine Optical Buoy (MOBY) rotation in clear water to provide a water-leaving radiance time series for postlaunch vicarious calibration; the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS) to hold the relevant data; clearly defined SeaWiFS Ocean Optics Protocols (SOOP) for established data collection methodologies; annual SeaWiFS Intercalibration Round-Robin Experiments (SIRREXs) for intercomparing field and calibration equipment, and training scientific personnel; direct comparison to a national standard laboratory using the SeaWiFS Transfer Radiometer (SXR); a portable field source, called the SeaWiFS Quality Monitor (SQM), for monitoring the temporal stability of the calibration of field instruments; a highly accurate atmospheric correction algorithm designed for the SeaWiFS instrument response functions; bio-optical algorithms that encompass a broad range of bio-optical provinces; and satellite data processing, quality control, and analysis procedures for monitoring the postlaunch performance of the sensor and the validity of the derived products. The culmination of many of these activities is the deployment of the instruments and methodologies on Atlantic Meridional Transect (AMT) cruises between England and the Falkland Islands, a 13 000 km voyage spanning more than 100° of latitude, with a calibration and measurement accuracy that is on the order of 1%. The AMT Program is the primary product validation activity supported by the SeaWiFS Project. The AMT cruises also serve as a testbed for new technology development and have demonstrated that high quality bio-optical data can be routinely provided to the Project in near-real time.