The Cluster Magnetic Field Investigation (FGM) aims to accurately measure the magnetic field vector at the locations of the four Cluster spacecraft. This mission provides critical data for understanding the structure and dynamics of the Earth's magnetosphere, including processes such as magnetic reconnection, turbulence, and wave phenomena. The FGM instrument consists of two triaxial fluxgate magnetometers on each spacecraft, with a highly failure-tolerant design that allows for redundancy and reliable operation. The instrument operates in various modes, including FGMEXT, which enables data collection during periods of no telemetry acquisition. The FGM data are calibrated to ensure high accuracy, with measurements approaching 0.1 nT. Calibration techniques involve in-flight testing and ground-based comparisons to determine the transformation of raw measurements into physical units. The data processing software transforms the raw telemetry data into standard geophysical coordinate systems, enabling the analysis of magnetic field structures and dynamics. The FGM data are widely used by the Cluster scientific community for both independent studies and collaborative research. Initial results from the mission demonstrate the capability of the FGM to provide detailed insights into magnetospheric phenomena, including the magnetopause, bow shock, and cusp regions. The mission has also contributed to the understanding of solar wind interactions with the Earth's magnetosphere and the dynamics of space plasma. The FGM instrument has been successfully rebuilt and deployed, with support from various scientific and funding organizations. The data from the FGM are essential for achieving the scientific objectives of the Cluster mission, providing a comprehensive view of the magnetic field in the Earth's magnetosphere and its surrounding regions.The Cluster Magnetic Field Investigation (FGM) aims to accurately measure the magnetic field vector at the locations of the four Cluster spacecraft. This mission provides critical data for understanding the structure and dynamics of the Earth's magnetosphere, including processes such as magnetic reconnection, turbulence, and wave phenomena. The FGM instrument consists of two triaxial fluxgate magnetometers on each spacecraft, with a highly failure-tolerant design that allows for redundancy and reliable operation. The instrument operates in various modes, including FGMEXT, which enables data collection during periods of no telemetry acquisition. The FGM data are calibrated to ensure high accuracy, with measurements approaching 0.1 nT. Calibration techniques involve in-flight testing and ground-based comparisons to determine the transformation of raw measurements into physical units. The data processing software transforms the raw telemetry data into standard geophysical coordinate systems, enabling the analysis of magnetic field structures and dynamics. The FGM data are widely used by the Cluster scientific community for both independent studies and collaborative research. Initial results from the mission demonstrate the capability of the FGM to provide detailed insights into magnetospheric phenomena, including the magnetopause, bow shock, and cusp regions. The mission has also contributed to the understanding of solar wind interactions with the Earth's magnetosphere and the dynamics of space plasma. The FGM instrument has been successfully rebuilt and deployed, with support from various scientific and funding organizations. The data from the FGM are essential for achieving the scientific objectives of the Cluster mission, providing a comprehensive view of the magnetic field in the Earth's magnetosphere and its surrounding regions.