Jigme Khesar Namgyel Wangchuck succeeded his father, Jigme Singye Wangchuck, in December 2006 after the former monarch announced his abdication.
His formal coronation was postponed until after the country's transformation into a parliamentary democracy had been completed and did not take place until November 2008.
The new king, who was 26 when he became head of state, promised to build on his father's efforts to democratise Bhutan. His predecessor had already given up some of his absolute powers in 1998 and ruled in conjunction with the government, an assembly and a royal advisory council.
Jigme Khesar Namgyel Wangchuck studied in the US and at Oxford University, where he completed an MA in politics.
After graduating, the future monarch was encouraged by his father to travel abroad as an ambassador for the Bhutanese people.
He insisted that it was critically important for Bhutan to complete the process of becoming a constitutional monarchy, despite the reluctance of many Bhutanese to see a diminution of the monarch's powers.
In the run-up to the March 2008 elections, he travelled extensively around the country, encouraging people to take part in the vote.
The high regard in which the Bhutanese monarchy is held, and the former king's foresight in scaling back its powers, makes it unlikely that it will suffer the same fate as the royal family in Nepal.
In November 2011, the king married a commoner, 21-year-old student Jetsun Pema.
Hassanal Bolkiah, one of the world's longest-reigning monarchs, became sultan in October 1967 after his father, Sir Haji Omar Ali Saifuddin, abdicated. He was crowned in August 1968.
In 1991 he introduced a conservative ideology called Malay Muslim Monarchy, which presented the monarchy as the defender of the faith.
Apparently aimed at pre-empting calls for democratisation, it is said to have alienated Brunei's large Chinese and expatriate communities.
In September 2004 the sultan reopened Brunei's parliament, 20 years after it was suspended. Observers said it was a tentative step towards giving some political power to the country's citizens.
The sultan was born in 1946 and was educated in Malaysia and Britain. He has two wives and is one of the world's richest individuals.
The son of former king Norodom Sihanouk, King Sihamoni was sworn in as monarch on 29 October 2004. The former king abdicated because of poor health.
Born in 1953, he studied in Czechoslovakia. He left Cambodia for France after the fall of the Khmer Rouge in 1979. He is a trained classical ballet dancer.
Cambodia's kings once enjoyed a semi-divine status; today, the monarch's role is mainly ceremonial.
Akihito succeeded his father, Hirohito, in 1989. Under the 1947 constitution, Japan's emperors have a purely ceremonial role.
King Abdullah II, Jordan's monarch since 1999, has extensive powers: he appoints governments, approves legislation and is able to dissolve parliament.
Over the past few years, he has been facing growing demands for political reform, and following the popular uprising in Tunisia which led to the flight of the president in January 2011, King Abdullah dismissed his government and appointed the first in a series of prime ministers to oversee the introduction of political change.
Previously he had backed a 10-year programme for political, social and economic reform and supported a plan for elected local councils. Conservative legislators were apprehensive about the proposals.
Balancing diplomatic interests with domestic demands has been tricky for King Abdullah. The country's peace agreement with Israel and its close ties with the US are unpopular with many Jordanians.
In the wake of the November 2005 suicide bombings in Amman, the king declared that security and stability were top priorities and called for a strategy to deal with the "changed circumstances".
Abdullah is the eldest son of the late King Hussein and his British-born second wife, Toni. The couple divorced in 1972. Born in 1962 and educated in Britain and the US, he was named as crown prince shortly after his birth. The king transferred the title to his own brother, Hassan, in 1965, only to return it to Abdullah in 1999.
He is married to a Palestinian - an asset since most Jordanians are of Palestinian origin - and enjoys car racing, water sports and collecting antique weapons. He is a career soldier and once led Jordan's special forces.
King Letsie III succeeded his father, King Moshoeshoe, who was dethroned in 1990.
Five years later, after the return to civilian government and amid political instability, he abdicated and his father was reinstated as monarch.
Letsie III was restored as king in 1996 after his father died in a car accident. The monarch has no legislative or executive powers.
The ruling Grand Duke of Luxembourg succeeded to the title in 2000, on the abdication of his father Jean. He had already exercised the constitutional powers of the monarch since 1998.
Born in 1955, the future Grand Duke Henri studied politics in Geneva, where he met his Cuban wife Maria Teresa. He later underwent officer training at Britain's Sandhurst Academy.
He and his wife have five children, the eldest of whom, Prince Guillaume, is the heir-apparent.
The head of state's constitutional role is largely ceremonial, and in 2008 parliament further restricted it by rescinding the monarch's right to veto legislation.
This came after Grand Duke Henri, a staunch Roman Catholic, threatened to withhold approval from a planned bill legalising euthanasia.
Tuanku Abdul Halim was appointed the 14th paramount ruler, Malaysia's head of state, in December 2011. Having already held the post from 1970 to 1975, he is the first Malaysian king to be enthroned twice. He is also the oldest to be elected to the post at 83.
The post of paramount ruler is rotated every five years among the sultans of the nine Malay kingdoms.
King Willem-Alexander became the first Dutch male monarch in more than a century in April 2013 when his mother Beatrix abdicated to end a 33-year reign.
The generational change in the House of Orange-Nassau gave the Netherlands a moment of celebration and pageantry at a time of recession brought on by the European economic crisis.
The much-loved Beatrix ended her reign in a nationally televised signing ceremony as thousands of orange-clad people cheered outside. Her retirement followed in the tradition of her mother and grandmother.
Willem-Alexander's popular Argentine-born wife became Queen Maxima and their eldest daughter, Catharina-Amalia, became Princess of Orange and first in line to the throne.
The king, a water management specialist, has said he will bring a less formal touch to the monarchy.
Former prime minister Tupua was elected head of state by parliament for a five-year term in 2007 on the death of paramount chief Malietoa Tanumafili II, who had been in office since independence. With the election of Tupua, Samoa switched from being a constitutional monarchy to being a republic.
Born in 1938, Tupua is an academic historian and a member of one of the leading extended families of the country.
He entered parliament as a Christian Democrat MP in 1966, and served as prime minister in 1976-82 and deputy prime minister in 1985-88.
Spaniards credit King Juan Carlos with steering the country to democracy after the death of dictator Francisco Franco in 1975 and for saving Spain from a coup attempt in 1981.
However, the royal family's popularity has declined in recent years, amid criticism that it is out of touch with ordinary Spaniards as they struggle with a severe economic crisis.
King Juan Carlos's reputation has been tarnished by an ongoing corruption inquiry implicating his son-in-law, and by a luxury elephant-hunting safari in Botswana in April 2012 at a time of record unemployment in his country.
The Spanish chapter of the conservation group WWF removed King Juan Carlos as its honorary president after news of the elephant-hunting trip emerged.
Thailand is a constitutional monarchy.
Its king, Bhumibol Adulyadej, assumed the throne in June 1946 and is the world's longest-reigning monarch.
The royal family is revered by many Thais.
Thailand has strict lese-majeste laws, and those deemed to have offended the monarchy - which is still a powerful force in the country - are often dealt with severely.
George Tupou VI succeeded to the throne on the death of his elder brother, the bachelor George Tupou V, in March 2012.
The new King was Tonga's High Commissioner (ambassador) to Australia from 2008, having served as prime minister in 2000-2006. He quit as prime minister in the face of violent pro-democracy protests in February 2006, and was succeeded by Tonga's first non-royal and non-aristocratic head of government, Feleti Sevele.
The late King George Tupou V ushered in parliamentary democracy in 2010, ending centuries of feudal dominance of government.
Elizabeth II became queen of the United Kingdom of Great Britain and Northern Ireland in 1952 upon the death of her father, George VI.
She is the second longest serving head of state, after the Thai King Bhumibol Adulyadej, who was crowned in 1946.
She is also head of state of 16 independent countries including Canada and Australia.
As a constitutional monarch, her role in the legislative process is largely ceremonial.