The Grand Duke of Lithuania was the highest ruler of the Grand Duchy of Lithuania, a significant state in Eastern Europe from the 13th to the 18th centuries. This title was held by various monarchs, who played a crucial role in the region's politics, culture, and military affairs. The Grand Duchy expanded its territory significantly, becoming one of the largest countries in Europe during its peak. The Grand Duke often held the title concurrently with the King of Poland, especially after the Union of Lublin in 1569, which created a closer political alliance between the two states. Notable Grand Dukes include Gediminas, Vytautas the Great, and Sigismund II Augustus, who contributed to the cultural and political development of the region.