The Landgrave of Hesse-Kassel was a title held by the rulers of the Landgraviate of Hesse-Kassel, a territory in present-day Germany. This region was part of the larger historical area known as Hesse, which played a significant role in the political and military affairs of the Holy Roman Empire. The landgraves were influential during the 16th to 18th centuries, particularly in the context of the Protestant Reformation and the Thirty Years' War. The most notable landgrave was Wilhelm IX, who became known as King of Sweden and was a key figure in the development of the region's economy and military. The landgraviate was also known for its strong support of the arts and education, establishing institutions that contributed to the cultural landscape of Germany.