Observations of Very High Energy Gamma-Radiation from Markarian 421

Abstract:
This article intends to summarize the observational data on the first extragalactical object to be discovered as a VHE Gamma-ray source, the BL Lac object Mkn 421. After a historical overview, the present situation is reviewed. As an intermediate conclusion it can be stated that while the source's existence and strong variability is firmly established, its spectral behaviour at energies above approx. 3 TeV, especially in the quiescent state, is not known to a satisfactory precision.