"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." Context 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. See also Don't repeat yourself Feature creep If it ain't broke, don't fix it KISS principle Minimum viable product MoSCoW method Muntzing Overengineering Single-responsibility principle SOLID Unix philosophy Worse is better References Software development philosophies Programming principles