Peter Tennant Johnstone (born December 28, 1948) is Professor of the Foundations of Mathematics at the University of Cambridge, and a fellow of St. John's College.^[2] He invented or developed a broad range of fundamental ideas in topos theory. His thesis, completed at the University of Cambridge in 1974, was entitled "Some Aspects of Internal Category Theory in an Elementary Topos".^[3]

Peter Johnstone is a choral singer, having sung for over thirty years with the Cambridge University Musical Society and since 2004 with the (London) Bach Choir. Following a severe bout of COVID-19 in 2020, he was invited by the Bach Choir's musical director David Hill to provide the text for a new choral work about the pandemic which the Choir commissioned from the composer Richard Blackford; the piece, `Vision of a Garden', was performed at the Bach Choir's first post-lockdown concert in October 2021 in the Royal Festival Hall, london, and again in July 2023 in King's College Chapel, Cambridge^{[citation needed]}

He is a great-great-great nephew of the Reverend George Gilfillan who was eulogised in William McGonagall's first poem.^[4]

• Johnstone, Peter (1977), Topos Theory, Academic Press, ISBN 978-0-12-387850-2, Zbl 0368.18001.

— "[F]ar too hard to read, and not for the faint-hearted"^[5]

• Johnstone, Peter (1982), Stone Spaces, Cambridge University Press, ISBN 978-0-521-33779-3, Zbl 0499.54001.
• Johnstone, Peter (1987), Notes on Logic and Set Theory, Cambridge University Press, ISBN 978-0-521-33692-5.
• Johnstone, Peter (2002), Sketches of an Elephant: A Topos Theory Compendium. I, II, Oxford University Press, ISBN 978-0-19-852496-0, Zbl 1071.18002 (v.3 in preparation)