Image source, ReutersImage caption, William wore a dark green velvet blazer and polo neck, while Kate had on an Alexander McQueen dress made for her in 2011
The Duke and Duchess of Cambridge have arrived at London's Alexandra Palace for the first Earthshot Prize awards ceremony.
The prize was set up by Prince William to reward those trying to save the planet.
Five winners, each receiving £1m, will be announced at the ceremony, which is airing on BBC One at 20:00 BST.
Stars including Emma Watson, Dame Emma Thompson, David Oyelowo and Mo Salah will hand out awards.
Ed Sheeran, Coldplay and KSI will also perform music.
No celebrities have flown to London and guests were asked to "consider the environment" when choosing an outfit - with Watson wearing a dress made from 10 different dresses from Oxfam.
Image source, ReutersImage caption, Harry Potter actress Emma Watson has previously used her platform to call for climate change action
Image source, ReutersImage caption, Nigerian singer Yemi Alade and British actress Emma Thompson pose as they arrive at Alexandra Palace
The Earthshot prize's name is a reference to the "Moonshot" ambition of 1960s America, which saw then-President John F Kennedy pledge to get a man on the Moon within a decade.
Prince William has previously suggested that rather than people setting their sights on space tourism, the world's top minds should instead focus on saving Earth.
He hopes the prize will help to find solutions to the planet's environmental problems and tackle the pessimism that many feel about climate change.
Image source, ReutersImage caption, Selma actor David Oyelowo will be among the stars handing out prizes
Image source, PA MediaImage caption, The ceremony is being presented by Dermot O'Leary and Clara Amfo
Image source, ReutersImage caption, Liverpool FC striker Mo Salah poses as he arrives at the awards ceremony
The prize - launched a year ago - will give £1m to five projects that provide inspiring and innovative solutions to problems facing the planet each year from now until the end of the decade.
Fifteen finalists have already been selected, including a gorilla conservation project, a 14-year-old girl from India who has designed a solar-powered ironing cart and the government of Costa Rica for its project paying local citizens to restore natural ecosystems.
The overall winners will be selected from five different categories, which are: Protect and Restore Nature; Clean Our Air; Revive our Oceans; Build a Waste-free World; Fix our Climate.
Judges include broadcaster Sir David Attenborough, actress Cate Blanchett and singer Shakira.
Among the celebrities attending Sunday night's ceremony is Love Actually actress Dame Emma who criticised throwaway culture as she made her way to the event.
"If we had shown my parents how people live (today) how they will wander down the streets and coffee cup, immediately throw it away, eat, throw away, everything throwaway, they would've gone, 'What's going on?'" said Dame Emma.
The ceremony is being broadcast live on BBC One and iPlayer at 20:00 BST.
Protect and Restore Nature:
Pole Pole Foundation, Democratic Republic of Congo
A community-led model of conservation that protects gorillas and local livelihoods.
The Republic of Costa Rica
A scheme paying local citizens to restore natural ecosystems that has led to a revival of the rainforest.
An online platform connecting and empowering local conservation projects.
China's first public environmental database enabling citizens to hold polluters to account.
A technology to create profitable products from agricultural waste and put a stop to the burning of crops.
Vinisha Umashankar, India
A 14-year-old innovator and activist who has designed a solar-powered ironing cart with the potential to improve air quality across India.
A coral farming project designed to restore the world's dying coral reefs.
Living Seawalls, Australia
Innovative tiles attached to sea walls create habitats for marine life to attach to.
A global conservation programme protecting 6.5m sq km of the world's ocean.
Build a Waste-Free World:
The City of Milan Food Waste Hubs, Italy
A city-wide initiative that has dramatically cut waste while tackling hunger.
A sanitation solution that converts human waste into safe products for local farmers.
A tiny water treatment plant that turns 98% of wastewater into clean water.
AEM Electrolyser, Thailand/Germany/Italy
An ingenious clean hydrogen fuel technology designed to transform how homes and buildings are powered.
Solar-powered energy capsules making electricity affordable and accessible in energy-poor communities.
The world's first peer-to-peer energy exchange network in a country on the front-line of climate change.