Diez an der Lahn (German pronunciation: [ˈdiːts an deːɐ̯ ˈlaːn]) is a town in Germany's Rhein-Lahn district in Rhineland-Palatinate, on the borders of Hesse.

Sitting on the confluence of the Lahn and Aar rivers, the town and the area have been inhabited by humans since the Stone Age.

Diez, in Rheinland-Pfalz, and the adjoining city of Limburg, in the state of Hessen, are so close that in modern times they have increasingly merged into a single urban area, although they remain historically and politically distinct.

The Lahn Valley serves as the boundary between the highlands north of the Westerwald with the forest of Taunus rising to the south.

This valley, the Diezer gate, begins in the Limburg Lahn basin and rises towards Fachingen in the Lower Lahntal.

[5][6] Other prehistoric evidence includes burial mounds and pottery finds of Latène which would seem to indicate primitive cultures present during the Celtic period.

The fertility of the Diez valley and its location at the confluence of the Lahn and the Aar clearly made it a natural place for primitive peoples to settle.

Other early mentions In the post-Carolingian period include a reference to one Diez in the area of Niederlahngau, ruled by the Conradines.

Early references show that Heinrich II of Diez (1145–1189) accompanied Frederick Barbarossa on his Italian campaign where he was involved in diplomatic negotiations.

By 1329, Ludwig of Bavaria bestowed the right of municipal law to Diez, and at that time the town was fortified with a wall with five entry gates.

His eldest son William the Silent inherited the principality of Orange in Southern France from his cousin René of Chalon, as well as from his father the vast properties of the House of Nassau-Dillenburg in the Netherlands, which Engelbert I of Nassau had received by marriage in 1403.

The early House of Orange-Nassau extinguished in the male line with William III of England (d. 1702), while the later House of Orange-Nassau (and the present Dutch royal family) descend in the male line from William I's younger brother John VI, Count of Nassau-Dillenburg, and subsequently from the latter's fifth son, Count Ernst Casimir of Nassau-Dietz.

Their grandson Johan Willem Friso (1687–1711) became Stadholder in Friesland and Groningen, and in 1702 became the heir of William III of England and thus the founder of the younger House of Orange-Nassau and of the Dutch royal family.

William IV became stadtholder of the Netherlands in 1747 and reunited all of the Dutch and German possessions of his family (except for Nassau-Weilburg) in his hand, styling himself Prince of Orange and Nassau.

William I of the Netherlands recovered his former counties in 1813, but gave Nassau-Diez, Nassau-Hadamar und Nassau-Dillenburg to Prussia, in exchange with Luxembourg, two years later.

In the course of municipal reform in 1969 Loreley circle the lower Lahn (located in St. Goarshausen) Rhein-Lahn-Kreis merged with the spa town of Bad Ems, which became the country seat.

A plaque on the Schlossberg (Castle Hill above the stairs) remembers the expulsion and deportation of Jewish children and their caregiver (s) on 20 August 1935.

At the northern edge of Diez is the baroque Schloss Oranienstein, the 1684 Princess Albertine Agnes (1634–1696) Ruins of the Benedictine Monastery Dierstein remain below.

Following the example of the North American concept of "Business Improvement District"s Diez in 2006 combined force to revive and redevelop the town's urban core.

The initiative group "Orange Table", by Diezer citizens, is a non-industrial concentration to develop the environment of the city and region Diez.

Numerous bus routes in the Rhein-Mosel (VRM) Diez provide public transport links with the surrounding area.