The ATLAS transition radiation tracker (TRT) is the outermost tracker of the inner detector. It is embedded in a 2 T solenoidal magnetic field provided by the ATLAS central solenoid. It consists of around 300000 polyimide straw tubes. Each is 4 mm in diameter, with a thin gold-plated tungsten wire stretched through the middle. The tubes are filled with a mixture of 70% xenon, 27% CO2, and 3% O2. The signal is created when a charged particle creates ionisation in the gas. Charge carriers are accelerated towards the wire by a 1500 V potential, leading to gas amplification by an avalanche of secondary ionisation, and eventually to an electrical signal in the wire, which is read out. The space between the tubes is filled with polymer fibres (in the barrel) or foils (in the endcaps). Highly relativistic particles produce transition radiation on the many material boundaries this creates. The soft X-ray transition radiation photons, typically of around 10 keV energy can be absorbed by xenon atoms in the tube gas, amplifying the ionisation signal. As the amount of transition radiation produced depends on the relativistic factor $\gamma = E / m$, the signal strength ("high-threshold" versus "low-threshold" hit) can be used to help distinguish the light electrons from hadrons, up to a particle energy of around 150 GeV.