KISS is an acronym for "Keep it simple, stupid" as a design principle noted by the U.S. Navy in 1960. The KISS principle states that most systems work best if they are kept simple rather than made complicated; therefore simplicity should be a key goal in design and unnecessary complexity should be avoided. The phrase has been associated with aircraft engineer Kelly Johnson (1910–1990). The term "KISS principle" was in popular use by 1970.  Variations on the phrase include "Keep it Simple, Silly", "keep it short and simple", "keep it simple and straightforward" and "keep it small and simple".
While popular usage has translated it for decades as "Keep it simple, stupid", Johnson translated it as "Keep it simple stupid" (no comma), and this reading is still used by many authors. There was no implicit meaning that an engineer was stupid; just the opposite.
The principle is best exemplified by the story of Johnson handing a team of design engineers a handful of tools, with the challenge that the jet aircraft they were designing must be repairable by an average mechanic in the field under combat conditions with only these tools. Hence, the "stupid" refers to the relationship between the way things break and the sophistication available to repair them.
The principle most likely finds its origins in similar concepts, such as Occam's razor, Leonardo da Vinci's "Simplicity is the ultimate sophistication", Mies Van Der Rohe's "Less is more", or Antoine de Saint Exupéry's "It seems that perfection is reached not when there is nothing left to add, but when there is nothing left to take away". Colin Chapman, the founder of Lotus Cars, urged his designers to "Simplify, then add lightness". Rube Goldberg's machines, intentionally overly-complex solutions to simple tasks or problems, are humorous examples of "non-KISS" solutions.
An alternative view – "Make everything as simple as possible, but not simpler" – is attributed to Albert Einstein, although this may be an editor's paraphrase of a lecture he gave.
A variant used in marketing is "keep it simple and straightforward". 
In film animation
Master animator Richard Williams explains the KISS principle in his book The Animator's Survival Kit, and Disney's Nine Old Men write about it in Disney Animation: The Illusion of Life, a considerable work of the genre. The problem faced is that inexperienced animators may "over-animate" in their works, that is, a character may move too much and do too much. Williams urges animators to "KISS".
In software development
- List of software development philosophies
- Minimalism (computing)
- Reduced instruction set computing
- Worse is better (Less is more)
- Don't repeat yourself (DRY)
- You aren't gonna need it (YAGNI)
- Rule of least power
- The Fox and the Cat (fable)
- Don't repeat yourself
- Occam's razor
- There's more than one way to do it
- W. Heath Robinson
- Worse is better
- You aren't gonna need it
- Sturgeon's Law
- The Routledge Dictionary of Modern American Slang and Unconventional English, Tom Dalzell, 2009, 1104 pages, p.595, webpage: BGoogle-5F: notes U.S. Navy "Project KISS" of 1960, headed by Rear Admiral Paul D. Stroop, Chicago Daily Tribune, p.43, 4 December 1960.
- The Concise New Partridge Dictionary of Slang, Eric Partridge, Tom Dalzell, Terry Victor, Psychology Press, 2007, p.384.
- Clarence Leonard (Kelly) Johnson 1910—1990: A Biographical Memoir (PDF), by Ben R. Rich, 1995, National Academies Press, Washington, DC, p. 13.
- Pit & Quarry, Vol. 63, July 1970, p.172, quote: "as in every other step of the development process, follow the KISS principle — Keep It Simple, Stupid."
- "Kiss principle definition by MONASH Marketing Dictionary". Dictionary.babylon.com. 1994-11-18. Retrieved 2010-04-18.
- "Kiss Principle".
- Ram B. Misra (2004), "Global IT Outsourcing: Metrics for Success of All Parties", Journal of Information Technology Cases and Applications, volume 6 issue 3, page 21. Online version. Retrieved 2009-12-19.