  Cosmic rays and air showers at ultra-high energy are unique tools to test the
validity of Lorentz invariance. A brief overview is given on such tests
focusing on isotropic, non-birefringent Lorentz violation (LV) in the photon
sector. Based on the apparent absence of vacuum Cherenkov radiation and photon
decay, the LV parameter $\kappa$ is bound to $-0.6 \cdot 10^{-20} < \kappa < 6
\cdot 10^{-20}$ (98\% CL). We report an updated limit from cosmic-ray photon
observations and preliminary results on testing vacuum Cherenkov radiation in
air showers.
