Prince William has opened the 21st World Scout Jamboree, marking the 100th anniversary of the movement's founding.
Around 40,000 young people from around the world are at the 12-day event at Hylands Park near Chelmsford, Essex.
A "tent town" is housing boys and girls from more than 160 countries on the 300-hectare site.
The Scouting Movement was founded by Lord Robert Baden-Powell in 1907, and the first camp was set up on Brownsea Island in Dorset.
The prince launched the festival with the Duke of Kent, who is president of the Scout Association, before being taken on a tour of the site.
The jamboree is the biggest event in the history of the scouting movement, according to the Scout Association.
The scouts will take part in traditional activities such as adventure training and community service, and meet scouts from other countries.
The prince was cheered by hundreds of children as he arrived by helicopter.
WORLD SCOUT JAMBOREE
Hylands Park covers an area the size of 120 football pitches
The restaurant can serve more than 6,000 people at a time
An international gathering to celebrate the 10th anniversary was called off because of WWI
The first jamboree at London's Olympia had 8,000 scouts from 34 countries
The word "Jamboree" comes from the Swahili "jambo" which means "hello"
Scout jamborees have been held every four years since 1920
He was presented with a traditional neckerchief commemorating the jamboree, which he hooked over his neck.
During the tour he met scouts from around the world who discussed issues including sustainable housing and HIV, before joining a drum band.
One of the scouts, Alec, explained what he wanted to get out of the event.
"Just meeting people from all across the world. There are scouts from nearly every part of the world here.
"Making friends with all of them and we are taking part in loads of different adventure activities with them so it's just going to be loads of fun."
A spokesman said the Duke of Kent and Prince William observed first hand how scouting has evolved.
He said: "They saw how scouting creates a better world through providing challenge and adventure to both young people and adult volunteers."
The prince was invited to open the event despite never having been a scout himself.