It was a big moment for a little royal, as he made his debut on the balcony of Buckingham Palace to help celebrate his great-grandmother's official birthday.
At just over one year old, Prince Louis more than rose to the occasion as he charmed the world with his first, very enthusiastic, royal wave.
The third child of the Duke and Duchess of Cambridge pointed to the fly-past overhead, waved at crowds below him and politely clapped the national anthem as he stole the show at Trooping the Colour yesterday, as the Royal family turned out to honour the Queen and her Armed Forces.
The Duchess of Sussex, who gave birth to her son Archie just over a month ago, made her return to public life for the family occasion, joining the carriage procession and balcony appearance alongside the Duke.
The Duchess, who missed the state visit of President Donald Trump this week because she was on maternity leave, looked well and happy, riding with the Duchesses of Cornwall and Cambridge, and Prince Harry.
Jack Brooksbank, who married Princess Eugenie last year, also made his first appearance at Trooping the Colour, while the Duke of York this year took a prominent role as Colonel of the Grenadier Guards.
Archie Mountbatten-Windsor, the five-week-old son of the Duke and Duchess of Sussex, was not seen in public at the ceremony, too small to appreciate the fly-past that was so entertaining for his cousins. There were unconfirmed rumours that he was inside the palace, close to his parents as they undertook their duties.
The balcony appearance, always a highlight of the Queen's official birthday parade, this year saw at least 44 members of the family squeeze together in a rainbow of colour to greet the large crowds along the Mall.
The Queen, wearing a pastel tweed dress in shades of pale pink, mint, yellow and gold, and the badge of the Brigade of Guards, took centre stage of course, surrounded by her family en masse.
The day's ceremonial duties began with the carriage procession from Buckingham Palace to Horse Guards Parade for the Trooping ceremony.
The Queen, travelling alone in the Scottish State Coach after her husband, the Duke of Edinburgh, retired in 2017, was accompanied by a Sovereign's Escort from the Household Cavalry, which is made up of the Life Guards and the Blues and Royals.
On horseback behind the Queen's coach were the royal colonels: the Prince of Wales, Colonel of the Welsh Guards; the Princess Royal, Colonel of the Blues and Royals; the Duke of Cambridge, Colonel of the Irish Guards; and the Duke of York, Colonel of the Grenadier Guards.
In the first Barouche carriage were the Duchess of Cornwall, Duchess of Cambridge - in pale yellow Alexander McQueen - and Duke and Duchess of Sussex, chatting animatedly as they waved to crowds.
The Duchess of Sussex's attendance had not been confirmed in advance, with questions over whether an appearance at the family event would only emphasise her absence during the Trumps' visit just days before.
Wearing a dress by Clare Waight Keller and hat by Noel Stewart, along with what appeared to be a third ring next to her engagement and wedding rings, she was seen on the balcony in conversation with fellow newlyweds Princess Eugenie and Jack Brooksbank.
In front of them, Isla and Savannah Phillips enjoyed themselves beating in time with the music on the plush red hangings of the balcony.
As has now become tradition, the public's first glimpse of the Cambridge children came through the windows of Buckingham Palace, as they peered curiously through the glass at the spectacle below them.
Prince Louis already looked to be in fine voice, his mouth wide open, as his older siblings stood on either side of him to watch their great-grandmother's carriage return through the gates of Buckingham Palace.
By the time they stepped on to the balcony, the trio seemed excited to watch the display overhead. Princess Charlotte looked up and waved wildly while Prince George shielded his eyes from the sun and their brother - carried by his father - waved, stretched, pointed and pulled faces at an amused Duchess of Cornwall. When the national anthem came to its rousing conclusion, Prince Louis - wearing a blue and white outfit, which appeared identical to that worn by Prince Harry to the event in 1986 - clapped the military band and cheering crowds.
As the day drew to a close, the ceremony was declared a "full success". The Queen beamed in appreciation as the public cheered.
The colour, or ceremonial regimental flag, being paraded this year was from the 1st Battalion Grenadier Guards, with the massed bands of the Household Division providing the musical elements and the King's Troop, Royal Horse Artillery firing a 41-gun salute in Green Park to mark the Queen's official birthday.
In a small but significant moment of history, Trooper Nina Croker became the first woman in the Household Cavalry to ride.
This year's fly past included Typhoon jets and Puma and Chinook helicopters, with the Red Arrows ending the display. Strong winds precluded other aircraft, including a Spitfire, Hurricane and Lancaster from the Battle of Britain Memorial Flight, from flying.
The Sunday Telegraph