LONDON (AFP) - Queen Elizabeth II has reigned through the decline of the British empire and seen her family mired in scandal, but Britain's longest serving monarch marks her 90th birthday on Thursday (April 21) as head of an institution in good health.
After 64 years on the throne, the queen is one of the most famous and respected women in the world and "embodies the history of the 20th century", according to historian Kate Williams.
The monarch, rarely seen without her signature hat and handbag, has met most of the major figures of recent decades, from India's first prime minister Jawaharlal Nehru to Japanese emperor Hirohito.
South Africa's late anti-apartheid icon Nelson Mandela called her "my friend", and her reign also spanned the construction and the destruction of the Berlin Wall.
The sprawling British empire of the Victorian era, on which it was said that the sun never set, has shrunk to a few remnants during her reign.
Yet even in far-flung parts of the world, "when people refer to the queen they almost always mean our queen", said John Major, one of the 12 prime ministers to have held office while she has been on the throne.
At home the House of Windsor survived the turbulence of the 1990s, with its royal divorces and the death of Diana, and won a new generation of supporters through its younger members, particularly Prince William and his wife Kate.
Britain's slender republican movement condemns a system based on inherited privilege, but support for the monarchy runs high at 76 per cent, according to a recent poll.
Elizabeth Alexandra Mary Windsor, known as "Lilibet", was born on April 21, 1926, with no expectation of being queen.
Her fate dramatically changed when her childless uncle Edward VIII abdicated in 1936 to marry Wallis Simpson, a twice-divorced American.
Princess Elizabeth's father inherited the crown as George VI and she suddenly became heir to the throne.
On her 21st birthday, she vowed to spend her life serving her country, but this promise was put to the test sooner than she could have imagined.
Her father died suddenly in 1952 at the age of 56, making Elizabeth queen at the age of 25.
She was by then married to Philip, a distant cousin, who she has described as "my strength and stay". They have four children: Charles (born in 1948) Anne (1950), Andrew (1960) and Edward (1964).
The queen's role is highly ceremonial and she remains above the fray of politics.
"Ma'am", as her subjects address her, also presides over the 53-nation Commonwealth, including 15 former colonies where she is still the sovereign, such as Australia and Canada.
Her reign has had plenty of ups and downs.
In 1981, her son and heir Charles married Diana in a fairytale wedding that quickly turned sour despite the birth of their sons William and Harry.
She called 1992 her "annus horribilis" when the marriages of three of her children - Charles, Anne and Andrew - fell apart and her castle in Windsor was badly burnt in a fire.
In 1997, the queen became the target of public anger following the death of Diana in a Paris car crash, accused of being out of touch for staying at her estate in Scotland while crowds of mourners gathered in London.
In the two decades since, though, there has been a remarkable turnaround for the monarchy - helped along by a powerful communications machine.
The queen has cut palace budgets and Prince William's 2011 marriage to Kate Middleton, a commoner, has helped create the image of a more modern monarchy.
Charles, 67, is increasingly taking her place at public engagements but experts agree that she will most likely hold on to the end and not follow her Dutch and Spanish royal peers by abdicating.
She has visited 117 countries, posed for 139 portraits and given thousands of speeches, though never an interview.
Her private life is largely a mystery, but her love of dogs - she has owned more than 30 Corgis - and of horses is well documented.
She still rides, reads the Racing Post newspaper's horse racing tips, and her favourite drink is said to be a heady Dubonnet and gin cocktail.