The Cluster Magnetic Field Investigation (FGM) aims to provide accurate measurements of the magnetic field vector at the locations of the four Cluster spacecraft, which are in orbit around the Earth's magnetosphere. The primary objective is to study three-dimensional aspects of space plasma phenomena, including boundary regions such as the magnetopause and bow shock, and small-scale processes like magnetic reconnection and turbulence. The FGM instrument consists of two triaxial fluxgate magnetic field sensors and an electronics unit, designed for high failure tolerance. It operates in various modes, including an Extended Mode (FGMEXT) that allows data collection during periods without telemetry acquisition. In-flight calibration techniques are used to determine the transformation parameters from raw measurements to physical units, ensuring accurate data interpretation. Initial results from the commissioning phase and subsequent observations show the instrument's capabilities in studying magnetospheric phenomena, such as cusp crossings and bow shock dynamics. The FGM data are widely used by the scientific community for collaborative studies and are distributed through the Cluster Science Data System (CSDS).The Cluster Magnetic Field Investigation (FGM) aims to provide accurate measurements of the magnetic field vector at the locations of the four Cluster spacecraft, which are in orbit around the Earth's magnetosphere. The primary objective is to study three-dimensional aspects of space plasma phenomena, including boundary regions such as the magnetopause and bow shock, and small-scale processes like magnetic reconnection and turbulence. The FGM instrument consists of two triaxial fluxgate magnetic field sensors and an electronics unit, designed for high failure tolerance. It operates in various modes, including an Extended Mode (FGMEXT) that allows data collection during periods without telemetry acquisition. In-flight calibration techniques are used to determine the transformation parameters from raw measurements to physical units, ensuring accurate data interpretation. Initial results from the commissioning phase and subsequent observations show the instrument's capabilities in studying magnetospheric phenomena, such as cusp crossings and bow shock dynamics. The FGM data are widely used by the scientific community for collaborative studies and are distributed through the Cluster Science Data System (CSDS).