The thesis tempts to shed light on the life of Anna of Celje, who entered history by marrying the Polish king and Lithuanian grand duke Władisław Jagiełło and therefore became the first queen from the Celje family. It describes the circumstances which led to the connection between the Counts of Celje and the Polish Piast dynasty, and argues that the marriage between Ana and Jagiełło was not a consequence of the elaborate plans of the Celje family, but rather the result of favourable circumstances. The previously overlooked testament written by Hermann II shows that Anna was firmly integrated in the court life of Celje as a child. The Polish king’s marriage proposal was based on his wish to eliminate any doubt into his authority and when Anna came to Poland, she was accepted as the legitimate heir to the Polish throne. The marriage linked the Counts of Celje with the Polish court, thus expanding the dynasty’s influence, contributing to Barbara of Celje and Sigismund of Hungary’s engagement and enabling the diplomatic activities of Hermann II of Celje in Polish lands. It seems that Queen Anna only played a minor role in Poland. However, the few existing sources show she had a certain amount of influence in public life. There is also some evidence of her political activity; which caused her to fall into disfavour with the Teutonic Order. The thesis aims to collect as many available narrative and documentary sources as possible; mostly from Poland since they are unknown in Slovenian historiography. On the basis of sources that can be found in Slovenia, the thesis also tries to shed light on various claims made by Polish historiographers. The ultimate goal of the thesis is to prepare a framework for Anna’s biography, which neither Slovenian nor Polish historians have written so far.