 The primary objective of the cluster mission is to accurately measure the magnetic field along the orbits of the four spacecraft. The magnetic field plays an active role in defining the structure and dynamics of magnetospheric phenomena on all scales, and it is a key constituent of the plasma in and around the magnetosphere. With the four-point measurements on cluster, it has become possible to study three-dimensional aspects of space plasma phenomena on scales comparable with the size of the spacecraft constellation and to distinguish temporal and spatial dependencies of small-scale processes. The instrumentation used to measure the magnetic field on the four-cluster spacecraft includes magnetometers and the performance of the operational modes used in flight has been evaluated. Preliminary in-orbit calibration results show that all components of the magnetic field are measured with an accuracy approaching 0.1 NT. Examples of the capabilities of the investigation include studying the tail current sheet, the duskside magnetopause and magnetosheath, the bow shock and the cusp. The data processing flow and implementation of data distribution to other cluster investigations and to the scientific community in general have also been described. This article was authored by Abelo, CM Carr, MH Acunya and others. We are article.tv, links in the description below.