It is the final race of the world championship - estimated to be worth Â£10m to the Welsh economy.
The official "shakedown" test drives got under way on Thursday morning before the ceremonial start at Llandudno, Conwy county.
The father of Colin McRae, the former champion killed in a helicopter crash in 2007, will drop the starting flag.
Former co-driver, Welshman Nicky Grist, will also be at the rally for the weekend as the event marks the contribution McRae made to rallying across the world - becoming the youngest ever driver to lift the world title in 1995.
Racing proper begins on Friday morning in the Hafren forest stage near Llanidloes, Powys.
The race ends on Sunday, with stages taking in the Denbigh Moors, Llandudno's Great Orme and a ceremonial finish at the Deeside service park in Flintshire.
The Welsh government has described the rally as one of Wales' flagship events and recently announced a three-year extension to its partnership with the race.
Tourism minister Ken Skates said: "The rally provides an ideal platform to showcase Wales' many assets, including our spectacular scenery, with last year's media coverage estimated to have reached an international audience of 60 million."
Thousands of spectators are crowding into north and mid Wales over the next four days for the Wales Rally GB.