Langen is a town of roughly 39,000 in the Offenbach district in the Regierungsbezirk of Darmstadt in Hesse, Germany. The town is between Darmstadt and Frankfurt am Main and part of the Frankfurt Rhein-Main urban area. Langen is headquarters to Deutsche Flugsicherung, and is also home to the Paul-Ehrlich-Institut, a Federal institute for the evaluation and supervision of sera and vaccines in Germany.