His/Her Illustrious Highness (abbreviation: HIll.H.) is the English-language form for a style used by various members of the European aristocracy.
It is used to translate the Middle High German word Erlaucht (German: erleuchtet), similar to later Durchlaucht reserved for the Princes of the Holy Roman Empire.
From the Early modern period, the style Erlaucht has been used by the members of those comital families (Reichsgrafen), who like the princes held the status of Imperial immediacy. They retained it even after the German Mediatisation of 1802/03, confirmed by the Bundesversammlung of the German Confederation in 1828. It was also adopted by the cadet members of some princely families like Colloredo-Mansfeld, Fugger, Khevenhüller, Salm, Schönburg, Starhemberg, Stolberg, Waldburg or Waldeck-Pyrmont. Mediate comital families were entitled to the style Hochgeboren.
It is sometimes used to translate the Russian word Ssiatelstvo (Сиятельство), a style used by members of some Russian princely families (also sometimes translated as Serene Highness).