The article presents the main areas of Marian Biskup’s research on the history of the Teutonic Order and its territorial rule at the Baltic Sea. Marian Biskup was interested in the Teutonic Order from the very beginning of his scientific career. He was an author of various analytical articles and comprehensive monographs based on extensive archival research. He also wrote works of historical synthesis embracing the whole historical process. The first part of the article described the research on the origin of the opposition of the Prussian estates and the incorporation of Prussia into the Kingdom of Poland, as well as the history of the Thirteen Years’ War. Next, the author characterized Marian Biskup’s contribution in the preparation of works of historical synthesis of the Monastic State of the Teutonic Order in Prussia. The third major theme researched by Professor Biskup was the origin of the transformation of the Monastic State into a secular state. The final part of the article referred to publications devoted to the history of Prussian towns and the military history of the Monastic State, the history of research on the Teutonic Order, the history of Livonia and the history of the church in Prussia.