In particle physics, tracking is the process of reconstructing the trajectory (or track) of electrically charged particles in a particle detector known as a tracker. The particles entering such a tracker leave a precise record of their passage through the device, by interaction with suitably constructed components and materials. The presence of a calibrated magnetic field, in all or part of the tracker, allows the local momentum of the charged particle to be directly determined from the reconstructed local curvature of the trajectory for known (or assumed) electric charge of the particle.