Intensity Interferometry is a technique that has been invented by Hanbury Brown and Richard Twiss in 1963 who constructed and operated an observatory in the years 1963-1974 in Narrabi/Australia. They measured the diameter of 32 bright stars for the first time. Nowadays, modern technology has advanced significantly and this old technology becomes interesting again as it can be realised with little technological effort and cost by double-using existing Cherencov telescopes. The optical resolution that can be achieved only depends on the distance of the telescopes as the technique is insensitive to atmospheric turbulences. Typical angular resolutions are between 0.2-1.4 mili-arcseconds with telescope distances up to about 100m. In future setups resolutions can also go a few micro-arcseconds using telescope distances of 10-20km. Cherenkov Telescopes consisting of large light collection mirrors and nano second precision photon detectors (PMTs) can be upgraded to perform real time numerical correlation calculations using powerful GPUs. In this way one can reach sensitivities up to magnitude 4-5 in about 2h observation time and in the near future, upgrades of up to magnitude 6-7 using single photon counting will be possible which will allow to perform interesting science. Here we summarise the current state of art as realised with the Cherenkov Telescope Array projects VERITAS, HESS and MAGIC/LST, as well as, future developments to achieve higher sensitivities.