This article was first published on Jan 23, 2015, and updated on June 9, 2016
Thailand will mark their ailing King Bhumibol Adulyadej's platinum jubilee on Thursday (June 9) with a religious ceremony and festivities, amid concerns over his health.
The 88-year-old received heart treatment on Tuesday and has been in hospital for over a year.
Just last month, the world's oldest reigning monarch - Britain's Queen Elizabeth II - celebrated her 90th birthday.
Here's a brief look at six elderly monarchs who are still in power.
1. Queen of the United Kingdom, Queen Elizabeth II (Elizabeth Alexandra Mary), 90
The Queen is Head of State of the United Kingdom and 15 other Commonwealth territories, including Canada, Australia and New Zealand. The elder daughter of King George VI and Queen Elizabeth, she was born in 1926 and has been queen for 65 years, from the age of 25.
The Queen has many different duties to perform every day. Some are public duties, such as ceremonies, receptions and visits within UK or abroad. Other duties include reading letters from the public, official papers and briefing notes.
Even when she is away from London, she receives official papers nearly every day of every year and remains fully briefed on matters affecting her realms.
While there is no one way to greet the Queen, many use the traditional way. Men bow their heads while women do a small curtsy. She is also the Supreme Governor of the Church of England.
The Queen has had her entire wardrobe designed specially. Angela Kelly is personal assistant and senior dresser to The Queen. Her role includes designing for The Queen, which she has done since 2002. Both old and new fabrics are used in the designing of her wardrobe.
2. Yang di-Pertuan Agong of Malaysia, Tuanku Abdul Halim, 88
The Sultan of Kedah made history by being elected twice as the Yang di-Pertuan Agong.
The ceremonial Head of State, Tuanku Abdul Halim first ascended the throne as the fifth Yang di-Pertuan Agong in 1970 at a time when the country was recovering from racial riots. He was re-elected in 2011.
According to the Constitution, the Monarch is elected on five-year terms.
The Malaysian Constitution specifies that the executive power of the Federal government is vested in the Yang di-Pertuan Agong and is exercised by him on the advice of the federal Council of Ministers.
He is a nature lover who has a mini zoo which houses various kinds of birds and animals, including horses, deer, crocodiles, monkeys and rabbits. The mini zoo is at Istana Anak Bukit in Alor Setar.
3. King Bhumibol Adulyadej of Thailand, 88
The Thai king has ruled since 1946 and is highly influential. He is a constitutional monarch on paper, but is seen as the Kingdom's highest moral authority and a father figure.
He is widely reverred by Thais, and has been instrumental in brokering peaceful solutions to past political crises and unrests in the nation.
He made a rare public appearance on Dec 14, 2015, after failing to show up for nationwide celebrations marking his birthday earlier that month. His birthday is celebrated as Father's Day, with festivities and ceremonies across the country.
He is the world's longest-serving current head of state and the longest-serving monarch in the Thai history. He is also an accomplished saxophonist, music composer and a champion sailor.
4. Emperor of Japan, Akihito, 82
The 125th emperor of his line according to Japan's traditional order of succession, he acceded to the throne in 1989.
In Japan, the Emperor is never referred to by his given name. He is referred to as "His Imperial Majesty the Emperor" which may be shortened to "His Imperial Majesty". His seat is called the Chrysanthemum Throne and sits in the Imperial Palace in Kyoto. The Constitution of Japan says that the Emperor is "the symbol of the State and of the unity of the people" and derives his position from "the will of the people with whom resides sovereign power".
Emperor Akihito was born in December 1933 to the great joy of the entire nation, as he was the first son of Emperor Showa and Empress Kojun. He has four elder sisters, one younger brother and one younger sister. He is the first Japanese crown prince to marry a commoner. He is the world's only monarch with the title Emperor.
5. King of Norway, Harald V, 79
The King took the throne upon the death of his father Olav V in 1991. The son of the then-Crown Prince Olav and of Princess Märtha of Sweden, King Harald was born at the Crown Prince Residence at Skaugum, Akershus, Norway. He was the first prince to be born in Norway in 567 years.
Like his father and grandfather before him, King Harald adopted the motto "We give our all for Norway". The King's duties are primarily ceremonial as the custodian of royal tradition. No acts of legislation or decisions approved by the Council of State are valid until sanctioned by the King and counter-signed by the prime minister. While the language of the Constitution still states that the executive power is vested in the King, the actual power lies with the Government.
The King has an important role to play during a change of government. Usually, the retiring prime minister will advise the King on who should assume the role of the new prime minister. If the parliamentary situation is unclear, however, the King must exercise his own discretion in selecting a new prime minister.
King Harald holds the rank of General in the Army and Air Force, and of Admiral in the Navy. He is the nation's highest-ranking officer.
6. King of Sweden, Carl XVI Gustaf, 70
He has been on the throne for 43 years. The fifth child and only son of Crown Prince Gustaf Adolf and Princess Sibylla of Sachsen-Coburg-Gotha, he is the seventh monarch in the country.
At the age of four, the current King became crown prince of Sweden when his great-grandfather Gustaf V died and was succeeded by the then 68-year-old Gustaf VI Adolf, the crown prince's grandfather. He ascended to the throne at the age of 27. His motto is "For Sweden - with the times".
As head of state, the King is regarded as Sweden's foremost unifying symbol. He is apolitical and without formal powers, based on the new constitution approved in 1974. He has a strong environmental commitment and is a recognised authority on environmental issues. He has even received the United States Environmental Protection Agency Award.
Sources: The Royal House of Norway, Official Website of the British Monarchy, The Imperial Household Agency, Official Sweden Government Website, The Star