Silesia (, also UK: , US: ) is a historical region of Central Europe that lies mostly inside the contemporary borders of Poland, with small parts in the Czech Republic and Germany. Its area is approximately 40,000 km2 (15,400 sq mi), and the population is estimated at around 8,000,000. Silesia is split into two main subregions, Lower Silesia in the west and Upper Silesia in the east. Silesia has a diverse culture, including architecture, costumes, cuisine, traditions, and the Silesian dialect (only in parts of Upper Silesia).
Silesia is along the Oder River, with the Sudeten Mountains extending across the southern border. The region contains many historical landmarks and UNESCO World Heritage Sites. It is also rich in mineral and natural resources, and includes several important industrial areas. The largest city and Lower Silesia's capital is Wrocław; the historic capital of Upper Silesia is Opole. The biggest metropolitan area is the Upper Silesian metropolitan area, the centre of which is Katowice. Parts of the Czech city of Ostrava and the German city of Görlitz are within Silesia's borders.
Silesia's borders and national affiliation have changed over time, both when it was a hereditary possession of noble houses and after the rise of modern nation-states, resulting in an abundance of castles, especially in the Jelenia Góra valley. The first known states to hold power in Silesia were probably those of Greater Moravia at the end of the 9th century and Bohemia early in the 10th century. In the 10th century, Silesia was incorporated into the early Polish state, and after its division in the 12th century it became a Piast duchy. In the 14th century, it became a constituent part of the Bohemian Crown Lands under the Holy Roman Empire, which passed to the Austrian Habsburg Monarchy in 1526. As a result of the Silesian Wars, the region was annexed by the German state of Prussia in 1742.
After World War I, the easternmost part of Upper Silesia was granted to Poland by the Entente Powers after insurrections by Poles and the Upper Silesian plebiscite. The remaining former Austrian parts of Silesia were partitioned to Czechoslovakia, forming part of Czechoslovakia's Sudetenland region, and are today part of the Czech Republic. In 1945, after World War II, most of Silesia was transferred to Polish jurisdiction by the Potsdam Agreement between the victorious Allies and became part of Poland, whose Communist government expelled most of Silesia's population. The small Lusatian strip west of the Oder–Neisse line, which had belonged to Silesia since 1815, remained in Germany.
As the result of the forced population shifts of 1945–48, today's inhabitants of Silesia speak the national languages of their respective countries. Previously German-speaking Lower Silesia had developed a new mixed Polish dialect and novel costumes. There is ongoing debate about whether the Silesian language should be considered a dialect of Polish or a separate language. The Lower Silesian German dialect is nearing extinction due to its speakers' expulsion.