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The night sky is filled with flashes of blue “Cherenkov” light, which last for just a few billionths of a second. Invisible to our eyes, these flashes result from particle cascades in the Earth’s atmosphere, triggered by the arrival of a high energy cosmic ray or gamma ray from space. They can be can be recorded using large reflecting Cherenkov telescopes equipped with fast photosensor cameras.  Studying the gamma-ray signals allows us to understand the particle acceleration processes occurring in some of the most extreme environments in the Universe – close to black holes, neutron stars, supernova remnants and relativistic jets. These gamma-ray Cherenkov telescopes are also the largest optical telescopes in the world, and can  be used to study rapid optical phenomena such as stellar intensity correlations and asteroid occultations. The field was pioneered at the Whipple Observatory in Arizona, which remains a very active center for its ongoing and future development. I will summarize some important results and the current status of the field, particularly in the USA.