Aßlar lies in the foothills of the Westerwald range and on the river Dill, which empties into the Lahn in neighbouring Wetzlar, about 5 km to the southeast.

Like many other places, Aßlar grew from a few great estates to about 5,000 by the time of the First World War, always forming a community with Klein-Altenstädten on the other side of the River Dill.

The municipal reforms of 1977 led to its amalgamation with Bechlingen, Berghausen, Bermoll, Oberlemp and Werdorf to produce today's town of Aßlar.

In the Bronze Age, people settled on the Schönbach, which flowed through the area, as this stream was more easily forded than the broader River Dill.

Tradition tells of a legend recounting how Werdorf got its name, namely: The municipal elections on 26 March 2006 yielded the following results: Note: FWG is a citizens' coalition.

Aßlar is today a small industrial town with well-developed infrastructure, where the firms Pfeiffer Vacuum and Manfred Huck are established.