Fans gathered at the site near Kinross to begin a weekend of music and partying despite the muddy and damp conditions.
Although the rain held off for the first day of the festival, the site was sodden from a week of torrential downpours.
Music fans were bracing themselves for worse to come.
Scott Fitzimmons, from Partick in Glasgow, who was gearing up for his third T in the Park, said the mud just added to the fun. The 23-year-old said: "We came up from Glasgow yesterday to get a good spot in the campsite and the ground wasn't that bad. Not great, but not bad.
"If you got upset about a bit of rain or mud then there would be no point in coming.
"When the conditions get really muddy and slidey, it's a good laugh – it's nearly as much fun as the music."
Last night, fans listened to Tinie Tempah headline the Radio 1/NME stage while Snow Patrol took the top slot on the main stage. Performances also came from Olly Murs, The Temper Trap, Professor Green, David Guetta and Rita Ora.
Saturday's headline slot is filled by reformed Mancunian indie band The Stone Roses while Kasabian are to close the festival on Sunday.
Welsh rockers Feeder were forced to cancel their performance after frontman Gary Nicholas contracted laryngitis.
The three-piece were due to perform on the Radio 1/NME stage. A statement from the band's management said: "It is with great regret that Feeder have had to cancel their main stage show at T in the Park due to band illness. Feeder look forward to playing to their Scottish fans on the Generation Freak Show tour in November."
More than 200 performers will take to the stage this weekend – not all of them pop and rock stars. Classical violinist Nicola Benedetti is to play the Main Stage of the festival, the first time a classical musician has taken a leading spot at the event.
Organisers warned festival-goers to prepare for any eventuality and bring both sunscreen and wellies.
More than 30,000 campers set up their tents on Thursday as the gates officially opened for the festival.
A further 55,000 are to join them during the rest of the weekend.
Superintendent Rick Dunkerley, T in the Park's event commander, said: "With 30,000 people arriving on Thursday and capacity building to 85,000 within the arena throughout Friday, Saturday and Sunday, the festival is not only one of the biggest and the best, but also one of the safest.
"In 2011, levels of crime were low and arrests down on the previous year.
"There was an increase in drugs seizures, which highlighted that drugs, and indeed crime in general, will not be tolerated at T in the Park."