Karak sits on the King's Highway about 140 km south of Amman. Public buses leave from Amman's Wahdat (South) Bus Station several times a day, taking roughly 2.5 to 3 hours; the fare is a few dinars. JETT-style tourist coaches do not serve Karak directly, so you may need to change at Tafilah if coming up from Aqaba or Petra.
Most travellers arrive by hire car. The fast option is the Desert Highway to Qatraneh and then the connector road west into Karak; the scenic option is the full King's Highway from Madaba, taking in Mount Nebo, Mukawir, and the Wadi Mujib gorge — allow a full day for that route. Shared taxis (servees) link Karak with Tafilah and Madaba and are quicker than the buses.