After the Battle of Endor, the Sith continued to cause problems for the fledgling New Jedi Order established by Jedi master Luke Skywalker. Using the deepest and darkest of the Sith arts, the ghost of Emperor Palpatine (Darth Sidious) eventually found its way to a new clone body. In an attempt to stop the new Emperor, Luke traveled to face him on the dark world of Byss. This resurrected Sith Lord managed to turn Skywalker to the Dark Side, but he was later saved by his Jedi sister, Princess Leia Solo.
Later, the ghost of ancient Sith Lord Exar Kun was awakened from its sleep deep within the Temples of Yavin IV when Luke set out to establish a new Jedi Academy in the old ruins. Kun's spirit corrupted two of Luke's students and trapped the Jedi Master in a spirit realm before Luke and his students were able to destroy the specter. They did this by using the light side of the Force to fight Kun in both the corporeal and spirit realms.
It's clear that as long as there are Jedi in the galaxy, there will also be the potential for the Sith to rise again.