The scatterometers on board the European remote sensing (ERS) satellites have proven their usefulness for weather analyses and forecasting, and operational programmes are being developed to provide routine scatterometer observations. The advanced scatterometer (ASCAT) on board the meteorological operational (MetOp) platforms is the follow on for European scatterometers. This paper describes the ASCAT system and its capabilities, including the instrument, calibration, ground processing, and products, and reviews the science associated with scatterometer measurements.