"You aren't gonna need it" (YAGNI) is a principle which arose from extreme programming (XP) that states a programmer should not add functionality until deemed necessary. Other forms of the phrase include "You aren't going to need it" (YAGTNI)  and "You ain't gonna need it" (YAGNI).
Ron Jeffries, a co-founder of XP, explained the philosophy: "Always implement things when you actually need them, never when you just foresee that you [will] need them." John Carmack wrote "It is hard for less experienced developers to appreciate how rarely architecting for future requirements / applications turns out net-positive."
YAGNI is a principle behind the XP practice of "do the simplest thing that could possibly work" (DTSTTCPW). It is meant to be used in combination with several other practices, such as continuous refactoring, continuous automated unit testing, and continuous integration. Used without continuous refactoring, it could lead to disorganized code and massive rework, known as technical debt. YAGNI's dependency on supporting practices is part of the original definition of XP.
- Extreme Programming Installed, Ronald E. Jeffries, Ann Anderson, Chet Hendrickson, 2001, 265 pages, p. 190, webpage: Books-Google-dIsC, quote: "YAGNI: 'You Aren't Gonna Need It.' This slogan, one of XP's most famous..., reminds us always to work on the story we have, not something we think we're going to need."
- Extreme Programming examined, Giancarlo Succi, Michele Marchesi, 2001, 569 pages, webpage: Books-Google-VSCh, quote: "XP says 'do the simplest thing that could possibly work ' because 'you aren't gonna need it'."
- Object-oriented & classical software engineering, Stephen R. Schach, 2007, 618 pages, p., webpage: Book-Google-hWwh, quote: "Two acronyms now associated with extreme programming are YAGNI (you aren't gonna need it) and DTSTTCPW (do the simplest thing that could possibly work)."
- Lowell Lindstrom; Carmen Zannier; Erdogmus, Hakan, eds. (2004). Extreme Programming and Agile Methods – XP/Agile Universe 2004: 4th Conference on Extreme Programming and Agile Methods. Lecture Notes in Computer Science. Berlin: Springer. p. 121. ISBN 3-540-22839-X.
- Martin Fowler; Kent Beck (8 July 1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley Professional, 431 pages, p. 68, webpage: BGoogle-1M. ISBN 978-0201485677. Quote: "you aren't going to need it".
- Mary Poppendieck; Tom Poppendieck (2003). Lean Software Development: An Agile Toolkit, p.59, webpage: BGoogle-hQ. Quote: "Kent Beck, Extreme Programming Explained, Chapter 17, uses the acronym YAGNI (You Aren't Going to Need It) for this practice and explains its rationale."
- Russ Olsen (2007). Design Patterns in Ruby, p.13, webpage: . ISBN 9780321490452. Quote: "This design principle comes out of the Extreme Programming world and is elegantly summed up by the phrase You Ain't Gonna Need It (YAGNI for short)."
- Ron Jeffries (April 4, 1998). "You're NOT gonna need it!". Retrieved 2007-11-07.
- Carmack, John [@ID_AA_Carmack] (18 June 2021). "It is hard for less experienced developers to appreciate how rarely architecting for future requirements / applications turns out net-positive. t.co/lusGdzkfd3" (Tweet). Archived from the original on 16 August 2022. Retrieved 2 December 2022 – via Twitter.