Only a part of Silesia lies in the Czech Republic. The whole Silesia used to be part of the Lands of the Czech Crown, along with Bohemia, Moravia, and Lusatia.
During the rule of Empress Maria Theresia, most of Silesia and the whole Lusatia was lost in the Austro-Prussian Wars, and later incorporated into Germany. After World War 2, this part of Silesia was ceded to Poland. Lusatia is mostly in Germany now.
The main centres of Czech Silesia are Ostrava and Opava. Along with a small area in the north-east of Moravia, it comprises the dialectial region of Lašsko. The Lach dialect is similar to Polish, mainly in phonology.