KISS, an acronym for "Keep it simple, stupid!", is a design principle first noted by the U.S. Navy in 1960. First seen partly in American English by at least 1938, 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. The term "KISS principle" was in popular use by 1970. Variations on the phrase include "keep it super simple", "keep it simple, silly", "keep it short and simple", "keep it short and sweet", "keep it simple and straightforward", "keep it small and simple", "keep it simple, soldier", "keep it simple, sailor", "keep it simple, sweetie", "keep it stupidly simple", or "keep it sweet and simple".

Clarence Leonard "Kelly" Johnson was an American aeronautical and systems engineer. He is recognized for his contributions to a series of important aircraft designs, most notably the Lockheed U-2 and SR-71 Blackbird. Besides the first production aircraft to exceed Mach 3, he also produced the first fighter capable of Mach 2, the United States' first operational jet fighter, as well as the first fighter to exceed 400 mph, and many other contributions to various aircraft.