On 9 September 2015, Queen Elizabeth II became the longest reigning sovereign in British history. This House of Lords Library briefing focuses on the Sovereign as part of the constitution of Parliament. It captures key aspects of the role of the Sovereign as it relates, in practice, to Parliament today, with an emphasis on the House of Lords. It does this through addressing a selection of the ceremonial and formal roles of the Sovereign: the State Opening of Parliament and its prorogation and dissolution; the calling of a new Parliament, in light of the Fixed-term Parliament Act 2011; Royal Commissions and royal assent; and Queen’s consent. The briefing also addresses a number of aspects of the Sovereign’s communications and relationship with Parliament more broadly, through Addresses and messages and the Palace of Westminster itself. It provides facts and figures relating to Queen Elizabeth II including: the number of Acts that have received Royal Assent during her reign, Addresses given by the Queen to both Houses in Westminster Hall, and the wording of Queen Elizabeth II’s first message to Parliament on her accession to the throne in 1952.