Performance of the ATLAS Transition Radiation Tracker read-out with cosmic rays and first high energy collisions at the LHC

The Transition Radiation Tracker (TRT) is the outermost of the three subsystems in the ATLAS Inner Detector. It contributes significantly to the precision of the momentum measurement of charged particles and to the identification of electrons. This note reports about the commissioning and performance of the TRT with cosmic rays and the first high energy collisions at the LHC.