The Lahn originates at the Lahnhof, a locality of Nenkersdorf, which is a constituent community of Netphen in southeastern North Rhine-Westphalia, near the border with Hesse.

Whereas the Sieg takes the shortest route to the Rhine (to the west), the Lahn first runs in the opposite direction, paralleling the Eder for many kilometers.

Above Bad Laasphe, where the river flows between the Rothaargebirge on the left (i.e. to the north) and the Gladenbach Uplands on the right, the Lahn Valley is simply considered part of these mountains.

Shortly after the village of Caldern (of Lahn Valley), the ridgeline of the Rothaargebirge on the north ends with the Wollenberg and that of the Gladenbach Bergland with the Hungert.

The Lahn leaves the Rhenish Slate Mountains for a long section and reaches the West Hesse Highlands, where it flows through the extreme south of the Wetschaft Depression, north of the Marburger Rücken.

Where the Wetschaft flows into it from the Burgwald forest in the north (near the Lahntal village of Göttingen), the Lahn immediately changes direction by 90° to the right.

The area between Heuchelheim, Lahnau, and the Wetzlar borough of Dutenhofen was to be completely mined and a water sports center with an Olympic-suitable rowing course built.

At this location, the valleys of the Lahn and Dill separate three parts of the Rhenish Slate Mountains from each other: the Gladenbach Bergland, the Westerwald to the northwest, and the Taunus to the south.

In the lower area, where the river turns again to the south, the Lahn is entrenched canyon-like below the level of the surrounding geographic trough.

At Aumenau in the municipality of Villmar, the course of the Lahn reverses to the west again and enters the fertile Limburger Basin, where the river is incised to a depth of about 50 metres (160 ft).

Then, after passing Nassau and Bad Ems, where, as in Fachingen, mineral springs (sources of Emser salt) can be found, it completes its 242 km (150 mi) run, entering the Rhine in Lahnstein, located five kilometers south of Koblenz at an elevation of 61 metres (200 ft).

The Lahn area was settled as early as in the Stone Age, as shown by archeological finds near Diez, in Steeden in the community of Runkel, and in Wetzlar.

These Lahnau sites have significant altered the current understanding of the history of the Romans east of the Rhine and north of the Limes.

The oldest mention of the staple right of Diez dates to the early 14th century and is an indication of significant shipping on the Lahn by that time.

In 1606, for the first time, the Lahn was deepened to allow small scale shipping and the lower reaches became navigable for four to five months of the year.

In the 17th and early 18th centuries, there were several initiatives of adjacent princes to further expand the Lahn as a waterway, but they all failed due to lack of coordination.

The newly created Duchy of Nassau eventually began work from 1808 under the Chief Construction Inspector of Kirn to make the Lahn fully navigable.

It was planned in the long run to make the Lahn navigable as far as Marburg and from there to construct a canal to Fulda to connect it with the Weser.

Little is known about the work that followed, but in the 1825 boatmen on the Lahn who shipped mineral water from springs in Selters and Fachingen addressed a letter of appreciation to the Nassau government in Wiesbaden for the rehabilitation of river systems.

Given the increasing ore mining in the Lahn Valley, officials from Nassau and Prussia in 1841 made an inspection trip along the river from Marburg to the Rhine.

The Prussians were the driving force behind river expansion projects, seeking to establish a connection between Wetzlar and their Rhine Province and to secure the iron ore supply for the growing industry in the Ruhr Valley.

The participating governments agreed to make the Lahn passable as far as Gießen for boats that were significantly larger than the existing vehicles on the river.

In Prussian territory, the work was largely completed by 1847, including construction of locks in Dorlar, Wetzlar, Wetzlar-Blechwalze, Oberbiel and Niederbiel.

In Nassau's territory, locks were built at Löhnberg, Villmar, and Balduinstein, as well as the greatest technical achievement: the Weilburg ship tunnel.

From the mouth upwards to Dehrn (of Runkel), Lahn-km 70 (above Limburg), the river is consistently passable for larger vessels, with locks operated by personnel.

The first partial section of a pilgrimage route, the Lahn-Camino on the left side of the Lahn, leads from Wetzlar Cathedral to Lahnstein via Castle Lahneck and the Hospital Chapel.

[2] Between the Lahn's source area in the Rothaargebirge and Gießen, all of the left tributaries are from the less mountainous parts of the West Hessian Bergland.

The right tributaries between the source area and the confluence of the Dill near Wetzlar come from the Gladenbach Bergland, while downstream they originate in the (High) Westerwald.

[3] Because the highest point of the Westerwald is near the Sieg, and especially because the Taunus is very close to the Main, both Mittelgebirge are each considerably more than half drained by the Lahn.

The river Emsbach runs through the Idstein Basin, which divides the (Hinter-) Taunus into two parts, while the Aar is central for the (Western and Eastern) Aartaunus.