Five years after ex-US president Bill Clinton spoke at the Hay festival, his deputy Al Gore is visiting one of the highlights of the literary calendar.
Al Gore has given up politics to campaign of global warming
The 19th Hay Festival which attracts up to 90,000 visitors to the small border book town has officially got under way.
Gore is one of the keynote speakers this year, with Nobel prize winner Seamus Heaney, Princess Michael of Kent and actor Alan Alda of US TV's Mash.
Katherine Jenkins opened the event and the Super Furry Animals will play.
Clinton famously called the literary festival the "Woodstock of the mind" when he made a pilgrimage to Hay in 2001.
The former president spoke on conflict resolution, and five years on Mr Gore, a guest on Monday, will use the Hay stage to warn of a "planetary emergency" over global warming.
Royal author Princess Michael is also visiting the festival
As well as attracting top writers - legendary American poet Maya Angelou is another former speaker - the event has become known as a platform for campaigners.
In 2005 Bob Geldof used his appearance at the Hay Literature Festival to rally demonstrators for last year's G8 Summit in Scotland.
Among the mix of celebrities who will be in conversation at this 10-day festival are Canadian writer Margaret Atwood, actress Sheila Hancock, who will talk about her book about her marriage to "Inspector Morse", the late John Thaw, the former SAS officer Andy McNab, and comedian Eric Sykes.
Frank Gardner, the BBC correspondent who survived an attack which killed his cameraman in Saudi Arabia, will give his perspective on the War on Terror.
Real life royalty will be there in the shape of Princess Michael of Kent, who has a new book out on royal paramours, while Liz Smith of The Royle Family fame will also talk about her autobiography.
Nobel prize winner Seamus Heaney is another keynote speaker
There are also a range of children's events, including talks by popular authors Jacqueline Wilson, Michael Morpurgo and Eoin Colfer.
A film festival runs alongside the literary one, and among the contributors will be Andrew Davies, whose latest production is the TV adaptation of The Line of Beauty.
This year too the festival hosts a number of rare public appearances by a selection of visual artists, furthering, according to the organisers, its reputation as a "festival of ideas".
Among them will be Sir Anthony Hodgkin, Turner prize winner Antony Gormley, and Turner prize nominee Cornelia Parker.
Hay has also developed new "offspring" including one in the Castillian city of Segovia in Spain this September and Spanish writers Fernando Savater, Carmen Posadas, Rafael Reign and Jorge Franco will be appearing at the Welsh event.
The festival this year moves to a new purpose-built site just outside the Powys town famous for its 41 bookshops, serving a normal round-the-year population of just 1500.