The royals were joined by President of France François Hollande as they arrived.
It's incredibly poignant to see a sea of poppies, filled with messages from the public, organised by the Royal British Legion.
This day 100 years ago, 1 July 1916, became one of the bloodiest in military history with almost 20,000 killed and double that number wounded in the first day of fighting.
William, Kate and Harry toured the centre where they visited the Somme tapestry.
A graphic mural lines the wall of the Thiepval centre, telling the history of the battles in the Somme countryside.
Prince William and President Hollande unveiled a plaque.
The Visitor Centre welcomes over 150,000 people to the Thiepval Memorial to the Missing each year. At the Visitor Centre the memorial is put in the context of the battlefield. Display panels in three languages — English, French and German — provide an overview of the course of the Great War from 1914-1918. Display panels focus on events during the Battle of the Somme in 1916 which occurred at the small village of Thiepval and its surroundings.
The new museum offers new galleries themed on the battles of the Somme and to the memory of the lost soldiers. The permanent 400m2 exhibition presents a great collection of artefacts, archaeological finds, multimedia displays and life-sized installations (replica of Charles Guynemer's plane). Like a window opening up onto the battle, Joe Sacco's vast 60 metre-long mural provides a visual account of the 1st July 1916.
The gallery dedicated to the Battle of the Somme opens onto a mural by illustrator Joe Sacco, it recounts hour by hour the horror of the tragic day of 1 July 1916. In the centre of the hall, a vast exhibition pit displays artefacts and archaeological finds of the First World War.
Afterwards, the young royals joined Prince Charles, the Duchess of Cornwall and the Duke and Duchess of Gloucester for the National Commemorative Event to mark the Centenary of the Battle of the Somme at the Commonwealth War Graves Commission Memorial to the Missing.
The royals greeted (soon to be former) Prime Minister David Cameron who has had a tumultuous week.
French and British schoolchildren marked each of the graves.
A sombre moment.
The royals joined political leaders including Prime Minister David Cameron, the Irish President Michael D Higgins, and French President François Hollande. Princess Anne's husband, Sir Timothy Laurence, was also there in his capacity as trustee of the Commonwealth Graves Commission.
An overview of the event.
The Kings Troop Royal Horse Artillery passed as the service of remembrance got underway.
Organisers said the event was designed to tell the story of the Battle of the Somme through music, readings and hymns accompanied by the BBC Orchestra.
The royals singing a hymn.
It was very much a case of teamwork on the Princes' behalf when it came to keeping the Duchess dry.
The royals walked past the graves of unknown soldiers killed without known burial sites.
Emily Andrews reported Kate giggled as she spoke French saying "It's about GCSE level" but was praised for "very good" language skills by a child she spoke to.
After the service, Kate chatted with Poppy Hodgson, from Hermitage Academy in Chester-le-Street. Ms Hodgson said: 'She said it was quite emotional being here and that they were really enjoying their visit. She said it was very moving.'
After, the royals attended a luncheon.
Kensington Palace shared this photo with the appropriate caption "At the going down of the sun and in the morning; We will remember them."
Kate wore a new lace dress for today's events. The knee-length frock is crafted of beige lace over black underlay and features a peter pan collar, three-quarter length sleeves and peplum detail. At this point the designer is unidentified.
Whilst we don't know the identity of the designer yet, we do know the lace was supplied by French lacemaker Sophie Hallette (with thanks to Sarah Beth).
The house of Sophie Hallette began its journey in 1887 and has gained recognition globally for making French lace and tulle. From McQueen to Dolce & Gabbana, their lace is used by high-end designers and favoured by celebrities.
The house first gained international recognition when Marilyn Monroe was seen in a bustier dress made of Hallette lace in 1953.
Kate has worn Sophie Hallette lace on a number of occasions - most notably her wedding day.
Most recently, Kate's white Dolce & Gabbana dress and grey Erdem coat both used Sophie Hallette lace.
As you can see, the popularity of the lace is considerable, meaning an array of designers could have created Kate's dress. Given Kate's close relationship with Sarah Burton and penchant for wearing McQueen on occasions of great importance I would lean towards that possibility. Equally, I wouldn't discount the likes of Dolce & Gabbana and Erdem. You can see the list of designers they supply here.
Kate brought back the black 'Lion Tamer' hat by Sylvia Fletcher for Lock & Co. hat she wore for Trooping the Colour and Sam Waley Cohren's wedding, both in 2011.
Kate accessorised with her Annoushka pearls and Kiki McDonough hoops.
The Duchess wore a poppy and a cornflower on her lapel today, as did the Duchess of Cornwall. The cornflower is the French equivelent of the poppy.
And completed the look with her black suede Mulberry clutch and black suede Gianvito Rossi pumps.
The next engagement in Kate's diary is the Art Fund Museum of the Year Prize on Wednesday. I also think it likely we'll see Kate at Wimbledon at some point over the next week. Until next time, wishing you all a wonderful weekend! :)