The article concerns the heretofore insufficiently examined question of the application of assorted languages in mediaeval Silesian documents. The first documents appeared in Silesia in the middle of the 12th century, and to the end of the following century they were all written in Latin. German-language documents were first issued at the beginning of the 14th century. The majority concerned town questions, and for a long time comprised a margin of the total output of the local chanceries. In about 1350 the chanceries of particular dukes began to use German-language documents to confirm a large part of various questions, as a rule resignations from landed estates. German also became more universal in the municipal chanceries and the private documents issued by knights. Up to the end of the 14th century, it dominated all secular document in Silesia. Latin documents continued to be written only for the clergy. The universal acceptance of the German language was by no means caused by a wish to oblige the chancery scribes. Multiple traces indicate that the latter found it even more difficult to write in German than in Latin. On the other hand, the German language suited the secular recipients as well as administrative and court instances, which required documents that would be understandable for all. Relations with the chancery of the Bohemian branch of the Luxembourg dynasty, which in the second half of the 14th century was simultaneously the Reich chancery, were also of considerable importance. The educated variety of the German language, used by the chanceries, spread across Silesia as well as central and eastern Germany. A language close to the local dialect was used only in private documents. The 15th century witnessed a further dissemination of German, now accepted even by Church chanceries, although basically only in connection with secular issues. In Upper Silesia, this development ran a different course: in about 1400 German prevailed as the language of documents, but from 1430 on the number of documents in Czech grew and in ca. 1470 won a leading rank. In its capacity as the official language, Czech survived until the 17th century. This situation corresponded to the linguistic relations in Upper Silesia whose population had retained its Slavonic character. The Polish language was still unsuited for use by chanceries, and even in Poland it was absent from official circulation. The extremely rare Polish-language documents did not appear in Silesia until the 16th century.