While the main stages do not open until Friday, many smaller venues put on live music, DJs and films for those arriving early.
The first performer on the official line-up is flautist Frei Zinger, on the Croissant Neuf Bandstand at 11:00 BST.
There will then be entertainment in the circus field, Groovy Movie Picture House and many of the dance arenas.
Some 177,000 people are due on site during the weekend. Tickets, costing Â£225, sold out in 26 minutes last October.
But the weather is not expected to be as bad as 2014, when performances were halted due to a lightning storm. According to BBC Weather, there will be some light showers on Friday morning and on Sunday.
This year's headliners are chart-topping singer Florence and the Machine, US rapper Kanye West and rock legends The Who.
US rock band Foo Fighters were due to headline on Friday, but were forced to pull out after frontman Dave Grohl broke his leg.
Florence Welch's last-minute promotion to the headline slot comes eight years after her first Glastonbury performance - a disastrous show in the Tiny Tea Tent for which her guitarist did not turn up until halfway through.
"If you had seen that performance at the Tea Tent there is no way you'd ever imagine I'd be headlining," she recently told The Times. "But I love Glastonbury. I'll do anything I can to help them out."
Organisers have not yet announced who will fill the vacant slot below Florence on Friday.
The following night, Kanye West's performance will be watched with interest after a petition calling on Glastonbury to drop him in favour of a rock band attracted 134,000 signatures.
Organiser Emily Eavis said she also received death threats from those who do not want him to play.
But Eavis wrote in The Guardian: "There is no question in my mind that we have got one of the greatest artists of his generation headlining, and we have no regrets at all about booking him."
Other names on the line-up include Motorhead, Mary J Blige, Burt Bacharach, Lionel Richie, Pharrell Williams, Mark Ronson and The Chemical Brothers.
More unusual bookings include Professor Stephen Hawking, who will appear in the Kidz Field, and the Dalai Lama.

The 22-year-old has made 13 appearances for Everton this season.
Everton manager Ronald Koeman last week said the player would be allowed to leave to "get game time".
The Spaniard joined Everton on loan from Barcelona for the 2013-14 season, making the transfer permanent in 2015 for £4.3m.
Milan originally announced the move via their official Twitter account on Friday, but it was quickly deleted.
This site is optimised for modern web browsers, and does not fully support your browser
Music fans have begun setting up camp at the Glastonbury Festival after the gates opened for this year's event.
[NEXT_CONCEPT]
Everton winger Gerard Deulofeu has joined Italian side AC Milan on loan until the end of the season.