Some argue that modern graffiti was born in Philadelphia, where the pioneers of wall-writing became citywide celebrities during the late 1960s and early 1970s. Graffiti eventually consumed the city, and a new mayor made it his top priority to fight back. In the process, the city's Mural Arts Program was born. Philadelphia is now a great outdoor museum. Yet graffiti still thrives, with new writers always ready to replace those who are caught or quit.

• Documentary