Happy birthday Queen Elizabeth II! The longest-reigning monarch in British history turns 90 years old on April 21.

The world is marking the occasion in all sorts of ways, from specially-decorated Boodles Mulberry Gin petit fours inspired by the iconic British sloe gin, to an edition of 10 new British stamps, including one each for the Queen, her son Prince Charles, her grandson Prince William, and her great-grandson Prince George, based on a photo taken by Ranald Mackechnie this past June.

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On the artistic side, the Fifield House Farm in Windsor erected giant Chia pet-esque terracotta busts of her and her husband, Price Philip, and artist David Parfitt teamed up with Kwik Fit to pay tribute to the Queen’s service during World War II as a mechanic in the Women’s Auxiliary Territorial Service, a branch of the British Army, by crafting her portrait from 800 British-made car parts.

An edition of 10 postage stamps honoring Queen Elizabeth II.
Photo: Courtesy of the Royal Mail.

There’s even a new patriotic shoe by designer Aruna Seth created in her honor; peep toe wedges decorated like the Union Jack and studded with almost 3,000 Swarovski crystals.

The Queen’s own personal celebrations will include lighting the first of over 900 celebratory beacons at her birthday party, and entertaining President Barack Obama at Windsor Castle on Friday, April 22.

In honor of the occasion, we spoke with royal photographer Mark Stewart, who has spent over 25 years photographing the Queen and her family in over 60 countries around the world. He shared some of his favorite shots of Her Majesty taken over the years, including one snapped just yesterday, April 20.

The Queen in her “the girls of Great Britain and Ireland tiara” in 2012.
Photo: Mark Stewart.

What is it like photographing the Queen?
The Queen is one of the easiest people to photograph, although she does have a range of smiles. You only see her truly beam at events like Ascot if her horses are running.

She is certainly happiest at events that involve one of her great loves: Horses, dogs, or the military. Without doubt, she’s a consummate professional and appears to have an amazing ability not to notice us photographers—yet she is always careful to make sure we get a picture.

It was rather a surprise though, when at a Diamond Jubilee Reception at Buckingham Palace, HM asked me and a colleague, “Do you gentlemen feel naked without your cameras?” She’s incredibly sharp and very quick-witted!

The Queen at the Windsor Horse Show in 2002.
Photo: Mark Stewart.

How many royal photographers are there, and how often is the Queen photographed at events?
There are a band of around 15 photographers regularly accredited to cover HM’s engagements, although in recent years around big events these numbers increase.

The Queen carried out 340 engagements last year. I would guess that she was photographed at about 200 of them. The remainder would be private audiences and meetings.

The Queen presents prizes for the Her Majesty the Queen’s Challenge for Services Team Jumping Competition at the Royal Windsor Horse Show.
Photo: Mark Stewart.

How has the Queen remained such a style icon over the years?
In reality HM’s style has not changed much in the 30 years I have been photographing her. She now has an in-house design team led by Angela Kelly LVO, personal assistant and senior dresser.

HM knows what works for her and sticks to tried and tested formulas, always wearing bright colors so she can be seen in a crowd and relying on trusty mid-height heels and favorite handbags. Hats tend not to be overly large, so that her face can be seen on engagements and in photographs.

The Queen is joined by the Duke of Edinburgh the day before her 90th birthday to reopen the bandstand in Windsor.
Photo: Courtesy of Mark Stewart.

What was the most exotic location you have ever photographed the Queen?
Photographing her in the Kremlin on her State Visit in 1992 was very historic. It was the first time a British Monarch had visited since her ancestors the Romanovs were killed in 1918.

What was the most memorable event with the Queen you were on hand to photograph?
A definite highlight was being one of only two photographers to capture HM’s audience with Pope John Paul II at the Vatican in 2000. Standing in the ailing Pope’s office with two of the world’s most respected and recognizable people was quite humbling and emotional, and if I’m honest, caused a butterfly or two!

The Queen bows her head as she joins other members of the royal family including Princess Margaret, Prince Edward, Viscount Linley, and the Duke and Duchess of York to watch the coffin of Diana, Princess of Wales pass Buckingham Palace on its way to her funeral at Westminster Abbey, September 6, 1997.
Photo: Mark Stewart.

What are some of the difficulties you have faced as a royal photographer? 
The challenges of Royal tours have changed over the years. In the early days we used to shoot on film and would have to develop films in our hotel bathrooms, then wire pics back to England. It was very complicated, particularly going through antiquated switchboards.

Nowadays the post-production captioning and editing can be a real headache with the huge number of images we shoot in digital format. One day of engagements can now produce thousands of images for me from which a selection has to be quickly made and sent to clients around the world.

This can be quite a challenge when you’re running on minimal sleep and traveling from place to place!

See more of Stewart’s photos of the Queen below:

The Queen is taken by surprise as she takes tea with Eton schoolboys at Guards Polo Club, 2003.
Photo: Mark Stewart.

The Queen Crying at the Field of Remembrance, Westminster Abbey, 2002.
Photo: Mark Stewart.

Queen Elizabeth II visits golden temple amritsar in India.
Photo: Mark Stewart.

Queen Elizabeth II reviews the company of Pikeman and Musketeers of the honorable artillery company at Armour House London.
Photo: Mark Stewart.

Queen Elizabeth II is joined by the Prince of Wales and Duchess of Cornwall and Duke and Duchess of Cambridge and Prince George, Prince Harry, James Viscount Severn, and the Duke of Edinburgh at Trooping the Colour 2015.
Photo: Mark Stewart.

Queen Elizabeth II’s horse Estimate wins the Queen’s vase on the fourth day of Royal Ascot and she is presented with her trophy by her husband the Duke of Edinburgh.
Photo: Mark Stewart.