"Kilroy was here" is an American popular culture expression that is believed to have originated during WWII, when a US shipyard worker began scrawling the phrase on ships he had inspected. US servicemen then took up the saying and began scrawling it all over the world, wherever they were stationed or encamped. The phrase is usually paired with a doodle of a man peeking over a wall. While the origins of the slogan are obscure, those of the cartoon are less so. It almost certainly originated as "Chad" in the UK before the war as a creation of the cartoonist George Edward Chatterton. Presumably, the two merged together during the 1940s, with the vast influx of Americans into Britain. The "Chad" cartoon was very popular, being found across the UK with the slogan "What, no …?" or "Wot, no …?" underneath, as a satirical comment on shortages and rationing. (One sighting, on the side of a British 1st Airborne Division glider in Operation Market Garden, had the plaintive complaint "Wot, no engines?"). Later, as the country began to prosper in the 1950s and 1960s, it became a feature of some forms of advertising, especially on posters touting home improvements etc. For instance in many areas of the country outdoor toilets were the norm, so a poster might say "Wot, no inside lav?" advertising indoor plumbing.

Kilroy was the most popular of his type in World War II, as well as today. Clem (Canadian), Overby (Los Angeles- late 1960s), Chad (British- WW II), and Mr. Foo (Australian- WW I & II) never reached the popularity Kilroy did. The ‘major’ Kilroy graffito fad ended in the 1950s, but today people all over the world scribble ‘Kilroy was here’ in schools, trains, and other similar public areas.

Kilroy is still known and used today by US Servicemen. He has been seen scribbled on barriers on Main Supply Routes (MSRs), inside Porta Potties at Camp Taqaddum, and on warehouses in Taji in Iraq