  Violations of spacetime symmetries have recently been identified as promising
signatures for physics underlying the Standard Model. The present talk gives an
overview over various topics in this field: The motivations for
spacetime-symmetry research, including some mechanisms for Lorentz breaking,
are reviewed. An effective field theory called the Standard-Model Extension
(SME) for the description of the resulting low-energy effects is introduced,
and some experimental tests of Lorentz and CPT invariance are listed.
