The first of tens of thousands of music fans have started arriving at RockNess on the shores of Loch Ness, near Inverness.
Last year, the three-day event attracted more than 35,000 revellers.
Headline acts over Friday, Saturday and Sunday include Fatboy Slim, The Strokes, Leftfield, Blondie and Doves.
Police have warned of heavy traffic on all routes to Inverness and the road leading to the site at the village of Dores.
Performances in the main arena were due to start at 1830 BST.
Fatboy Slim, aka DJ Norman Cook, will headline the opening night.
As well as warnings of congestion on roads, police have raised concerns of a "legal high" being used by revellers during the event.
The Association of Police Officers in Scotland (Acpos) said NRG-1, or Energy-1, was being marketed on websites as legal to use.
But the organisation said tests of samples had found the substance contained class B drugs.
Acpos said police would "rigorously target" drug misuse at RockNess.
Northern Constabulary the warning from Acpos had generated a number of worried calls from the public.
Insp Tom Ogilvie said: "In response to this warning, the force has been contacted by people expressing concern regarding the dangers of a relatively new psychoactive substance, referred to as NRG-1.
"This substance is reported to cause some disturbing effects, both physical and psychological, and its use has resulted in several admissions to accident and emergency departments."
He said families worried about a relative having the substance should contact their local police station, or Crimestoppers.