Graffiti (plural; singular graffiti or graffito, the latter rarely used except in archeology) is art that is written, painted or drawn on a wall or other surface, usually without permission and within public view. Graffiti ranges from simple written words to elaborate wall paintings, and has existed since ancient times, with examples dating back to ancient Egypt, ancient Greece, and the Roman Empire (see also mural).

Graffiti is a controversial subject. In most countries, marking or painting property without permission is considered by property owners and civic authorities as defacement and vandalism, which is a punishable crime, citing the use of graffiti by street gangs to mark territory or to serve as an indicator of gang-related activities. Graffiti has become visualized as a growing urban "problem" for many cities in industrialized nations, spreading from the New York City subway system and Philadelphia in the early 1970s to the rest of the United States and Europe and other world regions.