By-elections to the House of Lords occur when vacancies arise among seats assigned to hereditary peers due to death, resignation, or disqualification. Candidates for these by-elections are limited to holders of hereditary peerages, and their electorates are made up of sitting Lords; in most cases the electorate are those sitting hereditary peers of the same party affiliation as the departed peer.

Following the enactment of the House of Lords Act 1999, the number of hereditary peers entitled to sit in the House of Lords was reduced to ninety-two. The Earl Marshal and the Lord Great Chamberlain were entitled to sit ex officio; the remaining ninety were elected by all the hereditary peers before the passing of the reform.

Before the passing of the 1999 Act, the Lords approved a Standing Order stating that the remaining hereditary peers shall consist of:

• 2 peers to be elected by the Labour hereditary peers
• 42 peers to be elected by the Conservative hereditary peers
• 3 peers to be elected by the Liberal Democrat hereditary peers
• 28 peers to be elected by the Crossbench hereditary peers
• 15 peers to be elected by the whole House
  • By convention, whole-House elections elect members of the same affiliation as the departed peer.^[1] In some such elections there have been unsuccessful candidates of different affiliation.
• The holders of the offices of Earl Marshal and Lord Great Chamberlain to be ex officio members.^[2] The current holder of the office of Lord Great Chamberlain was previously elected to the House of Lords in a by-election.

Prior to November 2002, vacancies were automatically filled by the highest performing unnsuccessful candidate in the 1999 House of Lords election that elected the departed peer.

Elections must be held within three months of a vacancy occurring,^[3] and they take place under the Alternative Vote system for elections to individual vacancies^[4] and the Single Transferrable Vote for elections to fill multiple vacancies.^[5] All those on the Register of Hereditary Peers are eligible to stand, but only sitting (the "excepted") hereditary peers of the group in question may vote for the seats reserved for a single parliamentary group. This can result in very small electorates, such as only three voters in the 2003 election of Lord Grantchester.^[6][7] For the 15 peers elected by the whole House, life peers may also vote.

As of September 2023, there have been 18 by-elections^[8] among Conservative peers; 19 by-elections^[9] among Crossbench peers; 2 among Liberal Democrat peers; and 2 among Labour peers. In addition, there have been 14 by-elections by the whole House.

As of November 2023^[update], the party affiliations of the elected hereditary peers are as follows:^[10]

• One additional hereditary peer is an ex officio member of the Lords: Duke of Norfolk (Earl Marshal).