In the series, Don't Panic is a phrase on the cover of The Hitchhiker's Guide to the Galaxy. The novel explains that this was partly because the device "looked insanely complicated" to operate, and partly to keep intergalactic travellers from panicking. "It is said that despite its many glaring (and occasionally fatal) inaccuracies, the Hitchhiker's Guide to the Galaxy itself has outsold the Encyclopedia Galactica because it is slightly cheaper, and because it has the words 'DON'T PANIC' in large, friendly letters on the cover."
Arthur C. Clarke said Douglas Adams' use of "don't panic" was perhaps the best advice that could be given to humanity.
On 6 February 2018 SpaceX launched the Falcon Heavy rocket, carrying Elon Musk's Tesla Roadster which had "DON'T PANIC!" written on the screen on the dashboard as a reference to the series.