A personal union is a relationship of two or more sovereign states, which, through law, share the same person as their head of state. 

Personal unions can begin for very different reasons.  In many cases a princess who is already married to a king becomes pregnant, and their child inherits the crown of both countries.  However, a personal union sometimes was seen as a means against uprisings if a state wants an annexation of an other state.  The unions can be written down in a constitution, which clearly expresses that both states shall share the same person as head of state.  When that is not the case, a personal union can easily be broken.

Some examples

Andorra 
 Partial personal union with France since 1607 (the French president, and formerly the king of France, is one of the Heads of State in Andorra, the other co-head of state is the Bishop of La Seu d'Urgell, Catalonia, Spain.)

Belgium 
Personal union with the Congo Free State from 1885 to 1908, when it annexed the Congo Free State.

Bohemia 
 Personal union with Poland 1003 - 1004 (Bohemia occupied by Poles)
 Personal union with Poland 1300 - 1306 and Hungary 1301 - 1305 (Wenceslas II and Wenceslas III)
 Personal union with Luxembourg 1313 - 1378 and 1383 - 1388
 Personal union with Hungary 1419-1439 (Sigismund of Luxemburg and his son in law) and 1490 - 1526 (Jagellon dynasty)
 Personal union with Austria and Hungary 1526 - 1918 (except years 1619 - 1620)

Brandenburg 

 Personal union with the Duchy of Prussia from 1618, when Albert Frederick, Duke of Prussia died without male heirs and his son in law John Sigismund, Elector of Brandenburg became ruler of both countries. Brandenburg and Prussia maintained separate governments and seats of power in Berlin and Königsberg respectively until 1701, when Frederick William I consolidated them into one government.

Congo Free State 
Personal union with Belgium from 1885 to 1908, when Belgium annexed it.

Australia 
 Since 1941, upon the ratification of the Westminster Statute in 1942 - which ended the British Parliament's ability to legislate for Australia. The Australia Act of 1986, amongst other things, removed the Privy Council as the last court of Appeal in the Australian Judicial System. Elizabeth II of the United Kingdom serves, independently, as Queen of Australia, through her Vice-Regal Representative, the Governor-General, nominated by the Prime Minister.

Ireland 
 Personal union with the Kingdom of England from 1541, when the Irish Parliament proclaimed King Henry VIII of England to be also King of Ireland, to 1707 when the Kingdom of England and the Kingdom of Scotland both united by the Treaty of Union and were replaced by the Kingdom of Great Britain.
 Personal union with the Kingdom of Scotland from 1603, when King James VI of Scotland became King of England and King of Ireland to 1707, when the kingdom of England and the kingdom of Scotland united and were replaced by the Kingdom of Great Britain. 
 Personal union with the Kingdom of Great Britain from 1707 to 1800 when the two kingdoms were merged into the United Kingdom of Great Britain and Ireland. 
Personal union with Hanover from 1714 to 1800. 
 As a Commonwealth realm from 1922-1936/1949, when it became a republic (see Irish head of state from 1936-1949).

Denmark 
 Personal union with Norway from 1380 to 1814 (the Norwegian Riksråd was abolished in 1536)
 The Kalmar Union with Norway and Sweden from 1389 to 1521 (sometimes defunct)
 The kings of Denmark at the same time being dukes of Schleswig and Holstein 1460-1864. (Holstein being part of the Holy Roman Empire, now part of Germany)
 Personal union with Iceland from 1918 to 1944 when Iceland became a republic

England 
 Personal union with Ireland from 1541 (when Ireland was raised to the level of a kingdom) to 1707
 Personal union with Scotland from 1603 to 1707 (when they were joined together in the Kingdom of Great Britain)
 Personal union with the Netherlands from 1689 to 1702, with the Dutch Stadtholder also serving as the King of England, Scotland and Ireland.
The actual situation was slightly more complex with the Dutch provinces Holland, Zeeland, Utrecht, Gelderland and Overijssel entering into personal union in 1689 and Drenthe in 1696. Only 2 Dutch provinces never entered into the personal union: Friesland and Groningen.

France 
 Personal union with the Duchy of Brittany from 1491, when Duchess Anne of Brittany married King  Charles VIII of France under duress, to 1532 when the Duchy of Brittany was formally annexed to the Kingdom of France.
 Personal union with Navarre from 1589 to 1620, when Navarre was formally integrated into France. 
 Partial personal union with Andorra since 1607 (the French president is one of the Heads of State in Andorra)

Great Britain 
 Personal union with Ireland from creation in 1707 to abolition in 1801 (when they were joined together in the United Kingdom of Great Britain and Ireland)
 Personal union with Hanover from the accession of George I in 1714 to abolition in 1801

Hanover 
 Personal union with Great Britain and Ireland from 1714 to 1801
 Personal union with the United Kingdom from 1801 to 1837, when differing succession laws resulted in Queen Victoria ascending the British throne and her uncle Ernest Augustus that of Hanover.

Holy Roman Empire 
 Personal union with Spain from 1519 to 1556 under Charles V.

Hungary 
 Personal union with Croatia from 1102 to 1918.
 Personal union with Poland from 1370 to 1382 under the reign of Louis the Great. This period in Polish history is sometimes known as the Andegawen Poland. Louis inherited the Polish throne from his maternal uncle Casimir III. After Louis' death the Polish nobles (the szlachta) decided to end the personal union, since they did not want to be governed from Hungary, and chose Louis' younger daughter Jadwiga as their new ruler, while Hungary was inherited by his elder daughter Mary. Personal union with Poland in the second time from 1440 to 1444.
 Personal union with Bohemia from 1419 to 1439 and from 1490 to 1918
 Personal union with the Holy Roman Empire from 1410 to 1439 and from 1526 to 1806 (except 1608-1612)
 Personal union with Austria from 1867 to 1918 (the dual monarchy of Austria-Hungary) under the reigns of Franz Joseph and Charles IV (in fact it was rather a dynastic union, not a personal union.)

Iceland 
 Personal union with Denmark from 1918 to 1944 when the country became republic.

Ireland 
 Personal union with England from 1541 (when the Irish Parliament proclaimed King Henry VIII of England, King of Ireland) to 1707 (upon the formation of Great Britain). 
 Personal union with Scotland (and England) from 1603 to 1707 (when England and Scotland were joined together in the United Kingdom of Great Britain)
 Personal union with the Netherlands from 1689 to 1702, with the King of Ireland, Scotland and England also serving as Stadtholder of most of the provinces of the Netherlands. The actual situation was slightly more complex with the Dutch provinces Holland, Zeeland, Utrecht, Gelderland and Overijssel entering into personal union in 1689 and Drenthe in 1696. Only 2 Dutch provinces never entered into the personal union: Friesland and Groningen.
 Personal union with Great Britain from 1707 to 1801 (when they were joined together in the United Kingdom of Great Britain and Ireland).
 Personal union with Hanover from 1714 to 1801.
 Personal union with the United Kingdom of Great Britain and Northern Ireland from 1922 to 1937/1949 (see Irish head of state from 1936-1949).

Lithuania 
 Personal union with Poland from 1386 to 1569 - the Polish-Lithuanian Union. In 1569 transformed into a federation of the Polish-Lithuanian Commonwealth.

Luxembourg 
 Personal union with the Netherlands from 1815 to 1890

Navarre
 Personal union with France from 1589 to 1620, when Navarre was formally integrated into France.

The Netherlands 
 Personal union with England, Scotland and Ireland from 1689 to 1702, with the Stadtholder of most of the provinces of the Netherlands also serving as King of England, Scotland and Ireland. The actual situation was slightly more complex with the Dutch provinces Holland, Zeeland, Utrecht, Guelders and Overijssel entering into personal union in 1689 and Drenthe in 1696. Only 2 Dutch provinces never entered into the personal union: Friesland and Groningen. 
 Personal union with Luxembourg from 1815 to 1890.

Norway 
 Personal union with Sweden from 1319 to 1343
 Personal union with Denmark from 1380 to 1814 (the Norwegian Riksråd was abolished in 1536)
 The Kalmar Union with Denmark and Sweden from 1389 to 1521 (sometimes defunct)
 Personal union with Sweden from 1814 (when Norway declared independence from Denmark and was forced into a union with Sweden) to 1905

Poland 
 Personal union with Hungary from 1370 to 1382 and 1440 to 1444(see Hungary section above)
 Personal union with Lithuania from 1386 to 1569 known as the Polish-Lithuanian Union. In 1569 the union was transformed into a federation of the Polish-Lithuanian Commonwealth.
 Eastern part: Personal union with Russia from 1814 to 1832, known as Congress Poland; following the suppression of an army revolt, the territory was annexed outright by Russia.

Poland-Lithuania 
 Personal union with Sweden from 1592 to 1599
 Personal union with Saxony from 1697 to 1705, 1709 to 1733 and 1733 to 1763

Portugal 
 Iberian Union with Spain from 1580 to 1640 (under Philip II of Spain, his son and grandson)

Romania 
 Personal union between Wallachia and Transylvania from 1599 to 1600 under the rule of Michael the Brave
 Personal union between Wallachia, Moldavia and Transylvania from 1600 to 1601 under the rule of Michael the Brave
 Personal union between Wallachia and Moldavia from 1859 to 1862 under the rule of Alexander John Cuza

Scotland 
 Personal union with England and Ireland from 1603 to 1707 (when England and Scotland were joined together in the Kingdom of Great Britain)
 Personal union with the Netherlands from 1689 to 1702, with the King of Scotland, England and Ireland also serving as Stadtholder of most of the provinces of the Netherlands. The actual situation was slightly more complex with the Dutch provinces Holland, Zeeland, Utrecht, Gelderland and Overijssel entering into personal union in 1689 and Drenthe in 1696. Only 2 Dutch provinces never entered into the personal union: Friesland and Groningen.

Spain 
 Personal union with the Holy Roman Empire from 1519 to 1556 under Charles I (Charles V, Holy Roman Emperor)
 Iberian Union with Portugal from 1580 to 1640 (under Philip II of Spain, his son and grandson)

Sweden 

 Personal union with Norway from 1319 to 1343
 The Kalmar Union with Denmark and Norway from 1389 to 1521 (sometimes defunct)
 Personal union with Poland-Lithuania from 1592 to 1599
 Personal union with Norway from 1814 to 1905 (sometimes called Sweden-Norway)

United Kingdom of Great Britain and Ireland 
 Personal union with Hanover from 1801 to 1837, when differing succession laws resulted in Queen Victoria ascending the British throne and her uncle Ernest Augustus that of Hanover.

Political systems