The House of Lords, also known as the House of Peers, is the upper house of the Parliament of the United Kingdom. Membership is by appointment, heredity or official function. Like the House of Commons, it meets in the Palace of Westminster in London, England.

The House of Lords holds the government to account by scrutinising bills, alongside the House of Commons. Peers regularly review and amend bills before they reach Royal Assent, seeking agreement with the House of Commons on the final text (also known as 'ping pong'). While it is unable to prevent bills passing into law, except in certain limited circumstances, it can delay bills and force the Commons to reconsider their decisions. In this capacity, the House of Lords, being independent of the electoral process, acts as a check on the House of Commons. While members of the Lords may also take on roles as government ministers, high-ranking officials such as cabinet ministers are usually drawn from the Commons. The House of Lords does not control the term of the prime minister or of the government. Only the lower house may force the prime minister to resign or call elections.

While the House of Commons has a defined number of members, the number of members in the House of Lords is not fixed. Currently, it has 777 sitting members. The House of Lords is the only upper house of any bicameral parliament in the world to be larger than its lower house, and is the second-largest legislative chamber in the world behind the Chinese National People's Congress.

The King's Speech is delivered in the House of Lords during the State Opening of Parliament. In addition to its role as the upper house, until the establishment of the Supreme Court in 2009, the House of Lords, through the Law Lords, acted as the final court of appeal in the United Kingdom judicial system. The House of Lords also has a Church of England role, in that Church Measures must be tabled within the House by the Lords Spiritual.