The queen and her husband Prince Philip rode in a vintage horse-drawn carriage from Buckingham Palace, inspecting more than 1,000 soldiers in a traditional display of pomp and pageantry.
Many of the troops wore tall bearskin hats and red military tunics for today's ceremony.
"Trooping the Colour" originated from traditional preparations for battle, when flags were carried or "trooped" down the rank for soldiers to see.
The queen turned 88 on April 21, her actual birth date, which is usually celebrated in private. The monarch traditionally marks her birthday publicly in June.