The Grand Duchy of Lithuania was a European state from the 12th–13th century until 1569. Then it became a constituent part of Polish–Lithuanian Commonwealth until 1791 when the Constitution of May 3, 1791 abolished it in favor of a unitary state. It was founded by the Lithuanians, one of the polytheistic Baltic tribes from Aukštaitija. The duchy later expanded to include large portions of the former Kievan Rus' and other Slavic lands, covering the territory of present-day Belarus, Latvia, Lithuania and parts of Estonia, Moldova, Poland, Russia and Ukraine. At its greatest extent in the 15th century, it was the largest state in Europe. It was a multi-ethnic and multi-confessional state with great diversity in languages, religion, and cultural heritage.

Consolidation of the Lithuanian lands began in the late 12th century. Mindaugas, the first ruler of Grand Duchy, was crowned as Catholic King of Lithuania in 1253. The pagan state was targeted in the religious crusade by the Teutonic Knights and the Livonian Order. The multi-ethnic and multi-confessional state emerged only at the late reign of Gediminas and continued to expand under his son Algirdas. Algirdas's successor Jogaila signed the Union of Kreva in 1386, bringing two major changes in the history of Grand Duchy of Lithuania: conversion into Catholicism and establishment of a dynastic union between the Grand Duchy of Lithuania and the Kingdom of Poland.

Duchy of Lithuania (Latin: Ducatus Lithuaniae; Lithuanian: Lietuvos kunigaikštystė) was a state-territorial formation of ethnic Lithuanians, that existed from the 12th century until 1413. Most of the time it was a constituent part and a nucleus of the Grand Duchy of Lithuania. Other alternative names of the territorial formation, used in different periods, were Aukštaitija or Land of Lithuania (11th – 13th centuries), Duchy of Vilnius (14th – early 15th centuries), Lithuania Propria or simply Lithuania (in a narrow sense).

The formation emerged in the central and eastern part of the present-day Lithuania, known as Aukštaitija, as the Lietuva Land (Lithuanian: Lietuvos žemė). It is supposed to have formed in the central Lithuania on the left bank of the Neris River and swiftly expanded eastwards. This very land was mentioned in 1009 as Litua (see Name of Lithuania). The territory was ruled by senior dukes of an ethnic Lithuanian tribe, Aukštaitijans or "Lithuanians", in the initial meaning of the name.

Lithuania (ⁱ/ˌlɪθuːˈeɪniə/ or /ˌlɪθjuːˈeɪniə/; Lithuanian: Lietuva), officially the Republic of Lithuania (Lithuanian: Lietuvos Respublika) is a country in Northern Europe, the largest of the three Baltic states. It is situated along the southeastern shore of the Baltic Sea, whereby to the west lie Sweden and Denmark. It borders Latvia to the north, Belarus to the east and south, Poland to the south, and a Russian exclave (Kaliningrad Oblast) to the southwest. Lithuania has an estimated population of 3.2 million as of 2011, and its capital and largest city is Vilnius. The Lithuanians are a Baltic people, and the official language, Lithuanian, is one of only two living languages (together with Latvian) in the Baltic branch of the Indo-European language family.

For centuries, the southeastern shore of the Baltic Sea was inhabited by various Baltic tribes. In the 1230s the Lithuanian lands were united by Mindaugas, who was crowned as King of the Grand Duchy of Lithuania, the first Lithuanian state, on 6 July 1253. During the 14th century, the Grand Duchy of Lithuania was the largest country in Europe: present-day Belarus, Ukraine, and parts of Poland and Russia were territories of the Grand Duchy of Lithuania. With the Lublin Union of 1569, Lithuania and Poland formed a voluntary two-state union, the Polish–Lithuanian Commonwealth. The Commonwealth lasted more than two centuries, until neighboring countries systematically dismantled it from 1772 to 1795, with the Russian Empire annexing most of Lithuania's territory.