The Acts of Union (Scottish Gaelic: Achd an Aonaidh) were two Acts of Parliament: the Union with Scotland Act 1706 passed by the Parliament of England, and the Union with England Act passed in 1707 by the Parliament of Scotland. They put into effect the terms of the Treaty of Union that had been agreed on 22 July 1706, following negotiation between commissioners representing the parliaments of the two countries. By the two Acts, the Kingdom of England and the Kingdom of Scotlandwhich at the time were separate states with separate legislatures, but with the same monarchwere, in the words of the Treaty, "United into One Kingdom by the Name of Great Britain".The two countries had shared a monarch since the Union of the Crowns in 1603, when King James VI of Scotland inherited the English throne from his double first cousin twice removed, Queen Elizabeth I. Although described as a Union of Crowns, and in spite of James's acknowledgement of his accession to a single Crown, England and Scotland were officially separate Kingdoms until 1707 (as opposed to the implied creation of a single unified Kingdom, exemplified by the later Kingdom of Great Britain). Prior to the Acts of Union there had been three previous attempts (in 1606, 1667, and 1689) to unite the two countries by Acts of Parliament, but it was not until the early 18th century that both political establishments came to support the idea, albeit for different reasons.

The Acts took effect on 1 May 1707. On this date, the Scottish Parliament and the English Parliament united to form the Parliament of Great Britain, based in the Palace of Westminster in London, the home of the English Parliament. Hence, the Acts are referred to as the Union of the Parliaments.

The Parliament of Scotland (Scots: Pairlament o Scotland; Scottish Gaelic: Pàrlamaid na h-Alba) was the legislature of the Kingdom of Scotland. The parliament, like other such institutions, evolved during the Middle Ages from the king's council of bishops and earls. It is first identifiable as a parliament in 1235, during the reign of Alexander II, when it was described as a "colloquium" and already possessed a political and judicial role. By the early 14th century, the attendance of knights and freeholders had become important, and from 1326 commissioners from the burghs attended. Consisting of the "three estates" of clergy, nobility and the burghs sitting in a single chamber, the parliament gave consent for the raising of taxation and played an important role in the administration of justice, foreign policy, war, and all manner of other legislation. Parliamentary business was also carried out by "sister" institutions, such as General Councils or Convention of Estates. These could carry out much business also dealt with by parliament – taxation, legislation and policy-making – but lacked the ultimate authority of a full parliament.The Parliament of Scotland met for more than four centuries, until it was prorogued sine die at the time of the Acts of Union in 1707. Thereafter the Parliament of Great Britain operated for both England and Scotland after the creation of the Kingdom of Great Britain on 1 May 1707. When the Parliament of Ireland was abolished in 1801, its former members were merged into what was now called the Parliament of the United Kingdom.The pre-Union parliament was long portrayed as a constitutionally defective body that acted merely as a rubber stamp for royal decisions, but research during the early 21st century has found that it played an active role in Scottish affairs, and was sometimes a thorn in the side of the Scottish Crown.