  We review the phenomenology of models with flat, compactified extra
dimensions where all of the Standard Model fields are allowed to propagate in
the bulk, known as Universal Extra Dimensions (UED). UED make for an
interesting TeV-scale physics scenario, featuring a tower of Kaluza-Klein (KK)
states approximately degenerate in mass at the scale set by the inverse size of
the compactification radius. KK parity, the four-dimensional remnant of
momentum conservation in the extra dimensions, implies two basic consequences:
(1) contributions to Standard Model observables arise only at loop level, and
KK states can only be pair-produced at colliders, and (2) the lightest KK
particle (LKP) is stable, providing a suitable particle dark matter candidate.
After a theoretical overview on extra dimensional models, and on UED in
particular, we introduce the model particle spectrum and the constraints from
precision electroweak tests and current colliders data. We then give a detailed
overview of the LKP dark matter phenomenology, including the LKP relic
abundance, and direct and indirect searches. We then discuss the physics of UED
at colliders, with particular emphasis on the signatures predicted for the
Large Hadron Collider and at a future Linear Collider, as well as on the
problem of discriminating between UED and other TeV-scale new physics
scenarios, particularly supersymmetry. We propose a set of reference benchmark
models, representative of different viable UED realizations. Finally, we
collect in the Appendix all the relevant UED Feynman rules, the scattering
cross sections for annihilation and coannihilation processes in the early
universe and the production cross section for strongly interacting KK states at
hadron colliders.
