Historical region, northeastern Europe, on the Baltic Sea between the Oder and Vistula rivers. Occupied by Slavs and other peoples, it was ruled by Polish princes in the 10th century. German immigration into western and central Pomerania began in the late 12th century, and Polish dukes ruled this area under the suzerainty of the Holy Roman Empire until the 17th century. Eastern Pomerania was held by the Knights of the Teutonic Order from 1308 until it was reconquered by Poland in 1454. The elector of Brandenburg acquired the duchies in 1637. Prussia united western and central Pomerania in 1815 as the province of Pommern. Most of the area is now in Poland; its westernmost section is in eastern Germany.