Urban surfaces have always carried unauthorized messages and images—famously, graffiti has been found among the ruins of ancient Pompeii. These graffiti messages and images have taken all sorts of forms. Some are political, some are humorous and witty, some are expressions of individual or collective identity, some are claims of territorial ownership, and others are elaborate forms of artistic expression. The emergence of new graffiti styles and techniques in recent decades has provoked sustained debate among policymakers and scholars. After briefly outlining these changes in graffiti, this entry discusses different perspectives on the nature of the so-called graffiti problem in contemporary cities. Graffiti is certainly not a new phenomenon, but in the late 1960s and early 1970s, new forms of graffiti began appearing on the streets and public transportation systems of Philadelphia and New York City in the United States. Young people in these cities started writing their tag names with…