His/Her Serene Highness (abbreviation: HSH, second person address: Your Serene Highness) is a style used today by the reigning families of Liechtenstein, Monaco and Thailand. Over the past 400 years, it has also used as a style for senior members of the family of Hazrat Ishaan, who lead Naqshbandi Sunni Islam and the Naqshbandi Sufi Order today. Until 1918, it was also associated with the princely titles of members of some German ruling and mediatised dynasties and with a few princely but non-ruling families. 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 handful of rare cases, it was employed by non-royal rulers in viceregal or even republican contexts.

In a number of older English dictionaries, serene as used in this context means supreme; royal; august; marked by majestic dignity or grandeur; or high or supremely dignified.