Gamma-ray astronomy at the highest energies.

In 1983 ultra high energy γ-rays were detected from Cygnus X-3. This was of particular interest since it is the first identifiable source of cosmic rays. Since then, there has been an explosion of interest in the field with more than twenty air shower arrays planned or being used to search for γ-rays at energies above 10¹⁴eV. Observational techniques for both very high energy and ultra high energy γ-ray astronomy are described and existing and proposed experiments reviewed with particular reference to their geographical locations. The results obtained so far are summarized together with some of the models proposed to explain them.