Electromagnetic radiation/Gamma rays
Electromagnetic radiation: 11. Gamma rays
These have a wavelength of 0.01 nm or less, hence photon energies exceed 100keV. They were discovered by Henri Becquerel in 1898 as a form of radioactivity. (He discovered three types, which he called alpha, beta and gamma, hence the name. However, alpha and beta are particles, not electromagnetic rays.)
Gamma rays are even more dangerous to living things than are X-rays, and are sometimes used to sterilise equipment since they can kill virtually all bacteria and viruses.
There is no lower boundary to the wavelength hence no upper bound to frequency or photon energy. In October 2011, photons with energies exceeding 100 billion eV, i.e. over a million times the lower limit, were detected from the Crab pulsar.