The Perth Agreement was made in Australia in 2011 by the prime ministers of what were then the sixteen states known as Commonwealth realms, all recognising Elizabeth II as their head of state. The document agreed that the governments of the realms would amend their laws concerning the succession to their shared throne and related matters. The changes, in summary, comprised:

The ban on non-Protestants becoming monarch and the requirement for them to be in communion with the Church of England was not altered.

The Agreement was signed in October 2011 in Perth, Australia, which hosted the biennial Commonwealth Heads of Government Meeting (CHOGM). The institutional and constitutional principles of Commonwealth realms are shared equally as enacted in the Statute of Westminster 1931, which made the process of implementing the agreement lengthy and complex.

By December 2012, all the realm governments had agreed to enact it. New Zealand chaired a working group to determine the process. The Commonwealth realms – at the time including the United Kingdom, Canada, Australia, New Zealand, Jamaica, Barbados, the Bahamas, Grenada, Papua New Guinea, the Solomon Islands, Tuvalu, Saint Lucia, Saint Vincent and the Grenadines, Belize, Antigua and Barbuda, and Saint Kitts and Nevis – are independent of each other, while sharing one person as monarch in a constitutionally equal fashion. (Barbados has since become a republic.) The working group later affirmed that, across all these realms, appropriate laws were passed and came into effect, and then Deputy Prime Minister of the United Kingdom, Nick Clegg, reiterated this on 26 March 2015. Canada's law was challenged in court but has been upheld.

On the day the changes came into effect in March 2015, the first of the persons affected by the headline provision were the children of Lady Davina Windsor, the elder daughter of Prince Richard, Duke of Gloucester and Birgitte, Duchess of Gloucester; the succession positions of Lady Davina's son Tāne (born 2012) and daughter Senna (born 2010) were reversed, Tāne becoming 29th and Senna becoming 28th in line.

Succession to the throne in each of the Commonwealth realms is governed both by common law and statute. Under common law, the Crown was transmitted by male-preference primogeniture, under which succession passed first to the monarch's or nearest dynast's legitimate sons (and to their legitimate issue) in order of birth, and subsequently to their daughters and their legitimate issue, again in order of birth, so that sons always inherit before their sisters, elder children inherit before younger, and descendants inherit before collateral relatives.

Succession is also governed by the Acts of Union 1707, which restates the provisions of the Act of Settlement 1701, and the Bill of Rights 1689. These laws originally restricted the succession to legitimate descendants of Sophia, Electress of Hanover (the mother of George I), and debar those who are Catholics or who have married Catholics. The descendants of those who are debarred for being or marrying Catholics, however, may still be eligible to succeed. By a convention made explicit in the preamble to the Statute of Westminster 1931, the line of succession cannot be altered in any realm without the assent of the parliaments of the other 15 realms.

Challenges had been made against the Act of Settlement, especially its provisions regarding Catholics and preference for males. In Canada, where the Act of Settlement is part of Canadian constitutional law, Tony O'Donohue, a former Toronto city councillor, took issue with the provisions that exclude Catholics from the throne In 2002, O'Donohue launched a court action that argued the Act of Settlement violates the Canadian Charter of Rights and Freedoms, but the case was dismissed by the court.