  Quantum-gravity effects are expected to be suppressed by the Planck mass. For
experimental progress it is therefore important to identify potential
signatures from Planck-scale physics that are amenable to ultrahigh-precision
tests. It is argued that minuscule violations of Lorentz and CPT symmetry are
candidate signals. In addition, theoretical and experimental aspects of the
Standard-Model Extension, which describes the emergent low-energy effects, are
discussed.
