This article reviews many of the observational constraints on Lorentz symmetry violation (LV). We first describe the GZK cutoff and other phenomena that are sensitive to LV. After a brief historical sketch of research on LV, we discuss the effective field theory description of LV and related questions of principle, technical results, and observational constraints. We focus on constraints from high energy astrophysics on mass dimension five operators that contribute to IV electron and photon dispersion relations at order E/M-Planck. We also briefly discuss constraints on renormalizable operators, and review the current and future constraints on IV at order (E/M-Plank)(2).
