This paper describes the magnetometer instrument (MAG) to be flown on the Cassini spacecraft. The instrument consists of two magnetometers and an on-board data processing unit. One magnetometer is a Vector Helium device of a type previously flown on Ulysses and several other missions which has been modified to operate in a Scalar mode providing measurements of the magnitude of the local magnetic field with very small absolute error. This is the first flight of such an instrument. The other magnetometer is a Fluxgate device of similar design to that flown on Ulysses and on many previous missions but with newly developed electronics. Both magnetometers can provide vector measurements of the three components of the magnetic field with a sensitivity of about 10pT. The unique combination of Fluxgate/Scalar or Fluxgate/Vector Helium operation offers increased possibilities for scientific investigation of the magnetic field environment in the Saturnian system. The data processing unit contains dual redundant systems based on the 80C86 microprocessor. It features sophisticated on-board data processing, large internal data storage capability and internal failure detection and recovery, giving the instrument the capability to operate autonomously for extended periods. The MAG instrument development team is drawn from institutes in four countries reflecting the multi-national flavor of the Cassini/Huygens mission.