Thomas Erskine May, 1st Baron Farnborough, KCB, PC (8 February 1815 – 17 May 1886) was a British constitutional theorist and Clerk of the House of Commons during the Victorian era.

His seminal work, A Treatise upon the Law, Privileges, Proceedings and Usage of Parliament (first published in 1844) has become known as Erskine May: Parliamentary Practice or simply Erskine May: this parliamentary authority (book of procedural rules) is currently in its 25th revised edition (2019) and is informally considered part of the constitution of the United Kingdom.

May was appointed a Companion of the Order of the Bath (CB) on 16 May 1860[7] and promoted to Knight Commander (KCB) on 6 July 1866.

[9] In 1873, he was elected a bencher of the Middle Temple and awarded an honorary Doctorate of Civil Law by the University of Oxford in 1874.

His capacity for synthesis, and his ability to dovetail the various parts of the evidence ... carried him into a more profound and complicated elaboration of error than some of his more pedestrian predecessors ... he inserted a doctrinal element into his history which, granted his original aberrations, was calculated to project the lines of his error, carrying his work still further from centrality or truth.