His/Her Serene Highness (abbreviation: HSH) is a style used today by the reigning families of Liechtenstein and Monaco. It also preceded the princely titles of members of some German ruling and mediatised dynasties as well as some non-ruling but princely German noble families until 1918. It was also the form of address used for cadet members of the dynasties of France, Italy, Russia and Ernestine Saxony under their monarchies. Additionally, the treatment was granted for some, but not all, princely yet non-reigning families of Bohemia, Hungary, Italy, Poland, Romania and Russia by emperors or popes.
In a number of older English dictionaries, serene as used in this context means supreme, royal, august, or marked by majestic dignity or grandeur or high or supremely dignified. The style Serene Highness has an antiquity equal to that of Highness. However, in some, excluding the Latin language countries, Highness was considered a higher treatment than Serene Highness.
German-speaking lands 
The current, legal usage of the style in the German-speaking countries is confined to the Princely Family of Liechtenstein, the entirety of which bears the treatment.
The German term is Durchlaucht, a translation of the Latin (su)perillustris. This is usually translated into English as Serene Highness, however, it would be more literal to translate it as superior to, above, beyond or greater than illustrious, as it is an augmentation of Erlaucht ("illustrious"), which was borne by immediate counts (Reichsgrafen) of the Holy Roman Empire and, mediatised, of the German Confederation and the German Empire. More humorously, the 1911 edition of the Encyclopædia Britannica suggested another perfectly logical English version, "Your Transparency", based on a literal translation of German "durch", which can also mean "through", or "more thoroughly", and "-laucht", as in "Erlaucht" (illustrious), meaning radiant—in other words, something that lets light through: something transparent.
In 1375 Emperor Charles IV bestowed the nobiliary style Durchlauchtig upon the seven Prince-electors designated by the Golden Bull of 1356. As from 1664 Emperor Leopold I vested all Imperial Princes with the title, it became so common that the Electors like the Archdukes of Austria began to use the comparative address Durchlauchtigst. In the German Empire, the style of Serene Highness was usually held by princes of lower rank than those who were entitled to Highness (exceptions were the Wettin cadets of the Ernestine duchies), Grand Ducal Highness, Royal Highness, and Imperial Highness. Therefore, if a woman entitled to the treatment of Royal Highness married a man who was addressed only as Serene Highness, the woman usually retained her pre-marital style.
In 1905 Emperor Franz Joseph I of Austria granted the style of Durchlaucht to members of virtually every family which had held the title of prince in the former Holy Roman Empire, even if the family had never exercised sovereignty.
- Cadet branches of the sovereign Ernestine dukes (i.e., Saxe-Altenburg, Saxe-Meiningen, Saxe-Coburg and Gotha);
- Reigning Fürsten of the small German realms which survived the collapse of the Holy Roman Empire;:
- Mediatised princes and dukes (e.g., Ratibor), and, eventually, their family members;
- Morganatic princes, descended from reigning dynasties;
- Other non-reigning princes of the German nobility, but not (always) their cadets (e.g. the Princes von Bismarck, Carolath-Beuthen, Pless, Wrede).
By tradition, Durchlaucht is still attributed to the princely dynasties which were sovereign until 1917 or had been mediatised under the Austro-Hungarian Empire and German Confederation in 1815, although the usage has been unofficial since 1918.
Francophone Dominions 
There is some evidence that in pre-Revolutionary France, one entitled to be addressed as Serene Highness was considered to outrank someone who was merely addressed as Highness. Those members of the royal family who were not children or grandchildren of a king, i.e., the princes du sang, were entitled to be addressed as "Most Serene Highness" (son altesse sérénissime). The simple style of "Highness" (altesse) was claimed by the princes étrangers and the princes légitimés. In fact, these formal styles were seldom employed in conversation, since the princes du sang used unique styles (e.g. Mademoiselle, Monsieur le Prince), while the ducal peers, led by the proud Duc de Saint-Simon, avoided conceding the altesse to the princes étrangers and bâtards royals, prompting nobles of lesser rank to do likewise.
The reigning Prince of Monaco, Albert II, is addressed as His Serene Highness. His wife, Princess Charlene, and younger sister, Princess Stéphanie, are also referred to as Her Serene Highness. His elder sister and heiress presumptive, Princess Caroline, was also styled Her Serene Highness prior to her 1999 marriage, but is styled Royal Highness since then. In French, both male and female versions are Son Altesse Sérénissime (S.A.S.), which translates, literally, as "His/Her Most Serene Highness".
Children of the Savoy kings and crown princes of Italy were entitled to the treatment of Royal Highness, but more remote descendants in the male-line were Serene Highnesses by right (although often the style of Royal Highness was granted to them ad personam, e.g. the Dukes of Aosta, Dukes of Genoa.
The mediatised House of Thurn and Taxis, entitled to the Serene Highness treatment in the German Empire, had a non-dynastic cadet branch, the Dukes di Castel Duino, which obtained naturalisation in Italy in the 20th century. When incorporated into the Italian nobility, their use of the Serene Highness style was authorised by the Crown.
After 1886, great-grandchildren of Russian emperors in the male-line, and their descendants, were princes or princesses, and accorded the treatment of Serene Highness. The exception was the senior male by primogeniture in the patrilineal descent of each great-grandson, who retained the higher style of Highness.
Strictly, the Russian term, Svetlost, was an honorific used in adjectival form (Светлейший : Svetleyshiy) to refer to members of a select few of Russia's princely families (e.g. "The Serene" Prince Anatoly Pavlovich Lieven or "The Serene" Prince Dmitri Vladimirovich Golitsyn). However, when translated into non-Slavic languages and used in reference to a member of the imperial Romanov family, it was usually rendered as Serene Highness.
United Kingdom 
Queen Victoria elevated all of the princes who married her daughters to Royal Highness (except for Crown Prince Friedrich of Prussia, husband of Victoria, Princess Royal, who already possessed the HRH). This included HSH Prince Henry of Battenberg, husband of Princess Beatrice. That couple's children, born Serene Highnesses, were granted the style of Highness by their British grandmother.
Several morganatic branches of reigning German dynasties took up residence in the United Kingdom in the 19th century, where their German princely titles and style of Serene Highness were recognized by the sovereign. Included in this group were Princess Edward of Saxe-Weimar, Princess Victor of Hohenlohe-Langenberg, the dukes and princes of Teck, (Mary of Teck, George V's queen consort, was Her Serene Highness as a princess of Teck), and the Princes of Battenberg (Princess Andrew of Greece, mother of Prince Philip, Duke of Edinburgh, was "Her Serene Highness" prior to marriage). The Tecks descended from the royal house of Württemberg, and the Battenbergs descended from the Grand Dukes of Hesse and by Rhine.
During World War I, King George V revoked recognition of the style Serene Highness, hitherto used by some relatives of the British Royal Family who possessed German princely titles, but were British subjects. These were the Dukes and Princes of Teck and the Princes of Battenberg, who were compensated with peerages, viz. Marquess of Cambridge and Earl of Athlone for the Tecks, and Marquess of Milford Haven and Marquess of Carisbrooke for the Battenbergs.
- Dukes and princes of Arenberg
- Dukes of Beaufort-Spontin
- Dukes and princes of Croÿ
- Princes of Habsburg (archducal cadets resident in Belgium)
- Princes of Lobkowicz (resident in Belgium)
- Dukes and princes of Looz-Corswarem
- Princes of Stolberg-Stolberg
- Dukes of Ursel
Before 1947, the style His/Her Serene Highness (Ő Főméltósága, literally: "His/Her High Dignity") was in use in Hungary. Princes were entitled to use it, and between 1920 and 1944 the regent, Miklós Horthy, was styled as His Serene Highness the Regent of the Kingdom of Hungary (Ő Főméltósága a Magyar Királyság Kormányzója).
As the most powerful noble family in Portugal, the Dukes of Braganza had the official treatment of Serene Highness until 1640, when they mounted the Portuguese throne, thereby becoming entitled to the style of Royal Highness, however the infantas not in direct line for the throne of Portugal were titled as "His/Her Highness, the Serene Infante(a)
Agustin I de Mexico used that title also.
In 1807 Manuel de Godoy, Prince de la Paz, was accorded the style of Most Serene Highness, a treatment unique in that country.
See also 
- Style (manner of address)
- Use of courtesy titles and honorifics in professional writing
- Most Serene Republic
- Almanach de Gotha (Gotha: Justus Perthes, 1944), pages 111-113, 115
- Almanach de Gotha (Gotha: Justus Perthes, 1944), pages 73, 94, 97, 98, 121, 124, 126
- Velde, François. "Royal Styles and the uses of "Highness"". Heraldica.org. Retrieved 1-3-2009.
- Spanheim, Ézéchiel (1973). In ed. Emile Bourgeois. Relation de la Cour de France. le Temps retrouvé (in French). Paris: Mercure de France. pp. 107–108.
- Christoph Franke, ed. (1997). Genealogisches Handbuch des Adels Fürstliche Häuser Band XV (in German). Limburg an der Lahn: C. A. Starke. pp. 33–41.
- Almanach de Gotha (Gotha: Justus Perthes, 1944), page 104
- Almanach de Gotha (Gotha: Justus Perthes, 1944), pages 49
- Almanach de Gotha (Gotha: Justus Perthes, 1944), pages 170,190,248,372.