Kate and William's second child made hearts melt as she walked into the Royal Chapel at St James' Palace, holding her father's hand. His sister looked sweet as ever in a pretty blue floral print dress, a matching Alice headband and the most adorable blue Mary Jane shoes with white socks.
The Duke and Duchess of Cambridge started their own tradition when it came to christening their children, making the "personal decision" to have their first child, Prince George, baptised at the intimate Chapel Royal at St James's Palace.
They were joined by Prince Charles and the Duchess of Cornwall.
Louis, the fifth in line to the British throne who was born on April 23, was christened by the Archbishop of Canterbury at The Chapel Royal of St James's Palace in central London.
The Prince has promised to build an exclusive library for Louis in memory of Princess Diana and the 1926 AA Milne classic is the library's first book.
Following the 40-minute traditional religious service, guests attended a private tea which included slices of christening cake, from a tier taken from the duke and duchess' wedding cake.
Nicholas van Cutsem is the brother of William van Cutsem who was selected as a godparent for Prince George. Welby also led Markle's baptism service in the Chapel Royal and married the Duke and Duchess of Sussex in May at Windsor Castle. She is the daughter of the 8th Earl of Romney.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire.
From the duchess's side, Kate's parents Michael and Carole, brother James, pregnant sister Pippa and her husband James Matthews were there.