The Polish–Lithuanian Union was a relationship created by a series of acts and alliances between the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania that lasted for prolonged periods of time from 1385 and led to the creation of the Polish–Lithuanian Commonwealth, or the Republic of the Two Nations", in 1569 and eventually to the creation of a unitary state in 1791.
Important events in the process of union included:
- 1385 – Union of Krewo, a personal union that brought the Grand Duke of Lithuania, Jogaila, to the Polish throne
- 1401 – Union of Vilnius and Radom, which strengthened the Polish–Lithuanian union
- 1413 – Union of Horodło, a heraldic union that granted many szlachta rights to Lithuanian nobility
- 1432 (1432–34) – Union of Grodno, a declarative attempt to renew a closer union
- 1499 – Union of Kraków and Vilnius in which the personal union became a dynastic union and recognised the sovereignty of Lithuania and described relations between the two states
- 1501 – Union of Mielnik, a renewal of the personal union
- July 1, 1569 – Union of Lublin, a real union that resulted in creation of the semi-federal, semi-confederal Republic of the Two Nations (Polish–Lithuanian Commonwealth)
- May 3, 1791 – Polish Constitution of May 3, 1791: abolished the elective monarchy and turned it into a hereditary monarchy and established a common state, the Rzeczpospolita Polska (Polish Commonwealth), in its place. The Reciprocal Guarantee of Two Nations modified the changes by stressing the continuity of binational status of the state. The changes were reversed completely in 1792 under pressure from the Russian Empire.