  We review the basic features of transition radiation and how they are used
for the design of modern Transition Radiation Detectors (TRD). The discussion
will include the various realizations of radiators as well as a discussion of
the detection media and aspects of detector construction. With regard to
particle identification we assess the different methods for efficient
discrimination of different particles and outline the methods for the
quantification of this property. Since a number of comprehensive reviews
already exist, we predominantly focus on the detectors currently operated at
the LHC. To a lesser extent we also cover some other TRDs, which are planned or
are currently being operated in balloon or space-borne astro-particle physics
experiments.
