William IV (Guillaume Alexander; French: Guillaume Alexandre; 22 April 1852 25 February 1912) reigned as the Grand Duke of Luxembourg from 17 November 1905 until his death. He succeeded his father, Adolphe.

William was a Protestant, the religion of the House of Nassau. He married Princess Marie Anne of Portugal, believing that a Roman Catholic country ought to have a Roman Catholic monarch. Thus his heirs have been Catholic.

At the death of his uncle, Prince Nikolaus-Wilhelm in 1905, the only other legitimate male in the House of Nassau-Weilburg was William's cousin, Georg Nikolaus, Count of Merenberg, the product of a morganatic marriage. So in 1907, William declared the Counts of Merenberg non-dynastic, naming his own eldest daughter Marie-Adlade (18941924) as heir presumptive to the grand ducal throne. She became Luxembourg's first reigning grand duchess upon her father's death in 1912, and upon her own abdication in 1919, was succeeded by her younger sister Charlotte (18961985). Charlotte's descendants reign until the present day.

To date, William is the last monarch of Luxembourg to die whilst still on the throne.

Marie-Adélaïde (Marie Adelheid Thérèse Hilda Wilhelmine; 14 June 1894 – 24 January 1924), reigned as Grand Duchess of Luxembourg from 1912 until her abdication in 1919. She was the first Grand Duchess regnant of Luxembourg (after five grand dukes), its first female monarch since Duchess Maria Theresa (1740–1780, who was also Austrian Archduchess and Holy Roman Empress) and the first Luxembourgish monarch to be born within the territory since Count John the Blind (1296–1346).

Named as heir presumptive by her father Grand Duke William IV in 1907 to prevent a succession crisis due to his lack of a son, Marie-Adélaïde became Grand Duchess in 1912. She ruled through the First World War, and her perceived support for the German occupation forces led to great unpopularity in Luxembourg as well as neighbouring France and Belgium. In 1919, on the advice of Parliament and after enormous pressure from the Luxembourgish people, she abdicated on 14 January 1919 in favour of her younger sister Charlotte who managed to save the monarchy and the dynasty in a national referendum (28 September 1919).

After abdicating, Marie-Adélaïde retired in a monastery in Italy, before leaving due to ill health. She died of influenza in Germany on 24 January 1924, at the age of 29.