Hagen is the 41st-largest city in Germany. The municipality is located in the state of North Rhine-Westphalia. It is located on the south eastern edge of the Ruhr area, 15 km south of Dortmund, where the rivers Lenne and Volme (met by the river Ennepe) meet the river Ruhr. As of 31 December 2010, the population was 188,529.

The city is home to the FernUniversität Hagen, which is the only state-funded distance education university in Germany. Counting more than 67,000 students (March 2010), it is the largest university in Germany.

Hagen is home to the LWL-Freilichtmuseum Hagen, or Hagen Westphalian Open-Air Museum, a collection of historic industrial facilities. Trades such as printing, brewing, smithing, milling, and many others are represented, not only with static displays, but as living, working operations that visitors may in some cases participate in. It is located near the Hagen community of Eilpe.

The Historisches Centrum Hagen includes the city museum and Werdringen castle. In the Blätterhöhle cave in Hagen, the oldest fossils of modern people in Westphalia and the Ruhr were found. Some date to the early Mesolithic, 10,700 years B.C.E. It seems that the descendants of Mesolithic people in this area maintained a foraging lifestyle for more than 2000 years after the arrival of farming societies.