Bielefeld is a city in the Ostwestfalen-Lippe Region in the north-east of North Rhine-Westphalia, Germany. With a population of 341,755, it is also the most populous city in the administrative region of Detmold and the 18th largest city in Germany. The historical centre of the city is situated north of the Teutoburg Forest line of hills, but modern Bielefeld also incorporates boroughs on the opposite side and on the hills.