The Yost system was a star system in the galaxy. It was orbited by the planet Berzite, which itself had a moon.
In 3 BBY, after an extensive search for a secure world to house a new base, the Spectres managed to cross-reference Republic and Lasat star maps and found that a moon of Berzite in the Yost system was suitable for the purpose. However, the trip was delayed as the rebels' stolen Imperial cruiser-carrier did not have enough fuel to make the journey. Thus, the Spectres traveled to Horizon Base, an Imperial refueling station, to steal necessary fuel. There, one of the rebels, astromech droid Chopper, was separated and stowed away aboard an Imperial cargo ship. He soon befriended a protocol droid designated AP-5 and together, they managed to commandeer the vessel. After learning that the rebels planned on stationing their base in the Yost system, the Empire set a trap in the area. When AP-5 heard where the rebels intended to retreat, aware of the Imperial trap, he instead recommended the planet Atollon.