The House of Nassau is a diversified aristocratic dynasty in Europe. It is named after the lordship associated with Nassau Castle, located in present-day Nassau, Rhineland-Palatinate, Germany.
The lords of Nassau were originally titled "Count of Nassau", then elevated to the princely class as "Princely Counts" (in German: gefürstete Grafen, i.e. Counts who are granted all legal and aristocratic privileges of a Prince).
At the end of the Holy Roman Empire and the Napoleonic Wars, they proclaimed themselves with the permission of the Congress of Vienna "Dukes of Nassau", forming the independent state Nassau (capital city: Wiesbaden), a territory which is at present mainly part of the German Federal State of Hesse (Hessen), and partially of the neighbouring State of Rhineland-Palatinate (Rheinland-Pfalz).
The Duchy was annexed in 1866 after the Austrian-Prussian War as an ally of Austria by Prussia. It was subsequently incorporated into the newly created Prussian Province of Hesse-Nassau.