  The HEGRA system of imaging air Cherenkov telescopes has successfully
pioneered the stereoscopic observation technique of extensive air showers. The
observational method is briefly described and important results of recent
observations of sources of photons with TeV($=10^{12}$ eV)-energies are
summarized: The first detection of a TeV $\gamma$-ray signal from the
shell-type supernova remnant Cassiopeia A and preliminary results obtained from
the observation of strong variability of the extragalactic source Mkn 421
during observations carried out from February to May 2000.
