Queen Elizabeth II of the United Kingdom — 64 years
Queen Elizabeth II ascended to the British throne in 1952 following the death of her father George VI, making her the longest-reigning monarch in the world.
Last year, the 90-year-old surpassed the reign of her great-great-grandmother, Queen Victoria, which lasted from 1837 to 1901. Queen Elizabeth is the 45th longest-reigning monarch of all time.
As head of the Commonwealth, she is officially the queen of 16 countries, including Canada, Australia, New Zealand, and Jamaica. Between these countries, Queen Elizabeth has seen more than 160 different prime ministers take power during her reign.
Sultan Hassanal Bolkiah of Brunei — 49 years
Hassanal Bolkiah has ruled the tiny nation of Brunei since 1967, after his father abdicated the throne.
The sultan has reaped the benefits of Brunei's extensive petroleum and natural gas reserves. According to Forbes, he boasts a net worth of $20 billion, making him one of the wealthiest people in the world.
A testament to his fortune, Hassanal lives in a palace with nearly 1,800 rooms and more than 250 bathrooms. Tales of his extravagance abound.
As sultan, Hassanal has full executive authority over Brunei, which borders Malaysia and Indonesia on the island of Borneo. He's the only ruler Brunei has ever known since gaining independence from the UK in 1984.
Sultan Qaboos bin Said al Said of Oman — 46 years
Sultan Qaboos bin Said al Said, 75, became ruler of Oman when he launched a successful coup against his father in 1970.
Since taking the throne, Qaboos has pursued reforms in the small sultanate, according to AFP.
Oman sits across the Strait of Hormuz from Iran, and the sultan helped arrange secret US-Iran meetings that started in Muscat in 2012. Those meetings led to the first formal talks between the US and Iran since the 1979 Islamic Revolution, helping to pave the way for the 2015 nuclear deal.
In return for facilitating those meetings, however, the US reportedly overlooked Oman's worsening record on forced labor and human trafficking.
Qaboos' health has been failing in recent years, and since 2014 he has been hospitalized in Germany on several occasions. He has no designated heir.
Queen Margrethe II of Denmark — 44 years
Denmark's Queen Margrethe, 76, became ruler of Europe's oldest kingdom on January 14, 1972, after her father's death. She is the first woman to hold the Danish throne.
She is also a smoker, with no intention of quitting. When asked about stopping her habit by Der Spiegel in September, the queen replied, "If there is anything I do not have in mind, then that would be it."
King Carl XVI Gustaf of Sweden — 43 years
King Carl Gustaf turned 70 this spring, and in September he saw the 43rd anniversary on the Swedish throne.
A 2010 book claimed that he had visited seedy strip clubs while on trips abroad and that he had carried on an extramarital affair with a Swedish singer in the 1990s.
Critics said the allegation were poorly sourced, and the king's initial reaction, which, according to the Associated Press, was "rambling," didn't include a denial.
Many in the country saw that as a partial admission, and a leading newspaper called on him to step down.
The longest-ruling monarch ever: King Sobhuza II of Swaziland — 82 years
King Sobhuza II held the throne in Swaziland for 82 years, a reign believed to make him the longest-ruling monarch ever.
He was born on July 22, 1899, amid the Boer War in southern Africa. His father died when he was just 4 months old, "and a tribal council headed by his father's favorite wife chose the young Sobhuza as his successor from among a host of other sons," The New York Times reported upon his death on August 21, 1982.
He was sent abroad for his formal education, and his grandmother, with help from an uncle, acted as regent in his absence.
He took over as king in late 1921, though at the time his role was mostly ceremonial, as a British high commissioner ran the country at the time.