Duke of Estonia
The first duke of Estonia  (Danish: Hertug af Estland ) was appointed in 1220  by Valdemar II after Danish conquest of Estonia during Livonian crusade. The title was resumed by the kings of Denmark since 1269. For the period of 1266-82 during the reign of Queen Dowager Margaret Sambiria the title lady of Estonia (Latin: Domina Estonie) was used.
The dukes of Estonia rarely resided in Estonia. To govern the Duchy of Estonia the king of Denmark appointed together with royal counsellors the Lieutenant (Latin: Capitaneus) who resided in Reval. 
After the Livonian War Estonia became part of Swedish Empire and the title was gained by kings of Sweden. Crown Prince Gustav Adolph was Duke of Estonia 1607-1611 before he became King, but then officially abolished all Swedish duchies in 1618.
The title was resumed by the Russian tsars after the Great Northern War and Treaty of Nystad when Estonia became part of Russian Empire. The last duke of Estonia (Russian: Князь Эстляндский) was Nicholas II of Russia.
- Also known as Duke of Estland or Prince of Estonia or Lord of Estonia
- Skyum-Nielsen, Niels (1981). Danish Medieval History & Saxo Grammaticus. Museum Tusculanum Press. ISBN 978-87-88073-30-0.
- "Overture in Europe". estonica.org. Archived from the original on June 10, 2007. Retrieved 2008-10-05.
- Michael, Jones (2000). The New Cambridge Medieval History. ISBN 978-0-521-36290-0.
- Christiansen, Eric (1997). The Northern Crusades. Penguin. ISBN 978-0-14-026653-5.
- Moncure, James (1992). Research Guide to European Historical Biography. University of Michigan. ISBN 978-0-933833-28-9.
- Joubert, Carl (1905). Russia as it Really is. E. Nash.