Warp travel is the means by which starships cross the impossible distances of the galaxy, plunging out of real space and into the Warp to ride its currents. Because the immaterial dimension does not obey the laws of physical distance, a vessel that translates into it may traverse in weeks or months a journey that would otherwise take tens of thousands of years.
The passage is anything but safe. A ship in the Warp is protected from the horrors outside only by a shell of psychic force called a Geller field, and should that field fail, daemons may board and slaughter all aboard, or the vessel may be lost forever. To find their way, most Imperial ships rely on a Navigator, a mutant able to perceive the light of the Astronomican, the psychic beacon that serves as a guiding star across the dark.
Even a successful voyage is unpredictable. The tides of the Warp run fast and slow, so that time itself becomes uncertain, and two ships departing together may arrive years apart. When warp-storms rise, whole regions become impassable, cutting worlds off from the Imperium for generations. For all its terrors, warp travel remains the indispensable artery of galactic civilisation, without which no interstellar realm could exist at all.