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. At the end of the Holy Roman Empire, they proclaimed themselves Dukes of Nassau, forming the independent state Nassau in what is now the German states of Rhineland-Palatinate and Hesse. The duchy was annexed by Prussia in 1866, when it was incorporated into the new Province of Hesse-Nassau. Today Nassau is a geographical, historical and cultural region in Germany.
All Dutch monarchs since 1890 and the Grand Dukes of Luxembourg since 1912 have been descended in the female line from the House of Nassau. According to German tradition, the family name is passed only in the male line of succession. The house is therefore, from this perspective, extinct since 1985. However Dutch aristocratic customs (and Luxembourg's, which are based on the aforementioned) differ, and do not consider the House extinct. The Grand Duke of Luxembourg uses "Duke of Nassau" as his secondary title and a title of pretense.