Prince Philippe, Duke of Brabant
The heir apparent to the Belgian throne for 20 years, Philippe is married and has four children with his wife, Countess Mathilde d'Udekem d'Acoz. A controversial book last year claimed he was gay, an allegation he denied. A Lieutenant-General in the Belgian Army, he is widely regarded as unworthy to follow in the footsteps of his father, King Albert II.
Prince Alois and Princess Sophie of Liechtenstein
Alois Philipp Maria is the Hereditary Prince of Liechtenstein and Count Rietberg, while his wife is Sophie, Duchess of Bavaria. Alois, 44, has been Regent since 2004, waiting to take over the top job from his father, Hans-Adam II. Alois already has considerable power and threatened to veto an referendum on legalising abortion in 2011.
Prince Guillaume and Princess Stephanie of Luxembourg
The hereditary Grand Duke of Luxembourg is 31 years old, and has been heir apparent since 2000. He revealed in interviews on his 30th birthday that he had a "Dear Miss" on the go, and married Stephanie, a Belgian princess, in October last year. Their parents are seventh cousins. No surprise there.
Prince Felipe and Princess Letizia of Spain
Felipe, Prince of Asturias, 45, became heir apparent at 18. His father, the future King Juan Carlos I, was picked by General Franco to succeed him after his death, which came in 1975. After the 2004 Madrid bombings, Felipe took part in public protests against Basque separatists who, it later transpired, had nothing to do with the atrocity.
Prince Salman bin Hamad Bint Isa Al Khalifa and Hala bint D'aij Al Khalifa of Bahrain
The people of Bahrain are no doubt grateful for the many varied talents of their monarch-in-waiting, the First Deputy Prime Minister of the Kingdom and Deputy Supreme Commander of the Bahrain Defence Force. His wife, Hala bint D'aij Al Khalifa, didn't make the picture.
Prince Haakon and Princess Mette-Marit of Norway
The Crown Prince of Norway has been heir for 22 years. Aged 39, he is a four-star general and son of the current King, Harald V. He is also a cousin, in varying degrees of removal, to the monarchs of Belgium, Luxembourg, Denmark, Sweden, and the United Kingdom too. His wife, Crown Princess Mette-Marit, is also 39.
Prince Frederik and Princess Consort Mary of Denmark
The Crown Prince of Denmark, 44, has been waiting to be king since he was three. Interested in climate change and sustainability, he runs marathons in a little over three hours. To the delight of those who like this kind of thing, the Princess Consort, Mary, is an Australian he met in a pub in Sydney during the 2000 Olympics.
Prince Maha Vajiralongkorn and Princess Maha Chakri Sirindhorn of Thailand
The Royal Son and Crown Prince of Siam was yet to have his umbilical cord cut when he became heir apparent. Born in July 1952, he has been hanging around for only four months less than Prince Charles. His father, King Bhumibol Adulyadej, has been on the throne for 66 years.
Princess Victoria and Prince Daniel of Sweden
The 35-year-old princess, who is also the Duchess of Västergötland, is the eldest child of King Carl XVI Gustaf and Queen Silvia. She only became heir to the throne in 1979 when Sweden's laws of succession were amended along non-sexist lines. Her husband, Daniel, was a personal trainer before royal business took over.
King Willem-Alexander and Queen Maxima of the Netherlands
The Dutch monarchy has a unique tradition of abdication, rather than hanging on until the end. Willem-Alexander became the country's first king since 1890 when his mother Queen Beatrix, 75, stepped down on Tuesday after 33 years on the throne. Almost a million people took to the streets of Amsterdam to watch.
Prince Charles and Camilla, Duchess of Cornwall
The Prince of Wales is 64 and the longest serving heir apparent in the photo – just. He has been waiting patiently for "mummy" to move on since he was three, and has kept himself busy by gardening and writing to politicians. His wife, Camilla, has only been waiting since 2005 – a blink of an eye in comparison.
Prince Naruhito and Princess Masako of Japan
The monarch-in-waiting is a fairly straightforward character, but there is debate about whether his only child, daughter Aiko, should follow him, or if it should be his nephew. As hereditary ruling goes, Japan's gig is pretty much the cushiest, involving little more than apologising to other Asian countries for Japan's conduct during the war.