Hermann II, Count of Celje, was a Styrian prince and magnate, most notable as the faithful supporter and father-in-law of the Hungarian king and Holy Roman Emperor Sigismund of Luxembourg. Hermann's loyalty to the King ensured him generous grants of land and privileges that led him to become the greatest landowner in Slavonia. He served as governor of Carniola, and twice as ban of the combined provinces of Slavonia, Croatia and Dalmatia, and was recognized by a treaty in 1427 as heir presumptive to the Kingdom of Bosnia. The House of Celje's rise to power culminated in achieving the dignity of Prince of the Holy Roman Empire. At the peak of his power, he controlled two thirds of the land in Carniola, most of Lower Styria, and exercised power over all of medieval Croatia. Hermann was one of the most important representatives of the House of Celje, having brought the dynasty from regional importance to the foreground of Central European politics.

The Counts of Celje or the Counts of Cilli were the most influential late medieval noble dynasty on the territory of present-day Slovenia. Risen as vassals of the Habsburg dukes of Styria in the early 14th century, they ruled the County of Cilli as immediate counts (Reichsgrafen) from 1341. They soon acquired a large number of feudal possessions also in today's Croatia and Bosnia. They rose to Princes of the Holy Roman Empire in 1436. The dynasty reached its peak with Ulrich II of Cilli, but with his death in 1456 they also died out, and after a war of succession, the Habsburgs inherited their domains.