Track reconstruction is the task of finding and estimating the trajectory of a charged particle, usually embedded in a static magnetic field to determine its momentum and charge. It involves pattern recognition algorithms and statistical estimation methods. Depending on the physics goals, not all charged tracks have to be reconstructed. For instance, in many cases there is a physically motivated lower limit on the momentum or transverse momentum of the particles to be found. Other examples are short-range secondary particles, such as δ-electrons, that normally need not be reconstructed. It may also be useful to reconstruct electron-positron pairs from photon conversions in order to check the distribution of material in the detector. Track reconstruction frequently proceeds in several steps: 1. Pattern recognition or Track finding: Finds the detector signals (hits) that are generated by the same charged particle. 2. Track fitting: Estimates for each track candidate the track parameters and the associated covariance matrix. 3. Test of track hypothesis: Tests for each track candidate whether all hits do indeed belong to the track and identifies outliers.