Some see it as a way of reclaiming public space, to others it's reckless vandalism.
In the year Brad Pitt and Angelina Jolie spent £1m on works by British stencil artist Banksy, this two-part series explores the global boom in street art.
The series looks at the phenomenon of street art in two very different cities, New York and Sao Paulo.
Taking to the streets, Joby Waldman meets Elbow-Toe and Nunca. Each artist is a symptom of their age, and their neighbourhood.
Joby accompanies them while they work and meets their artistic circles, creating a highly personal audio portrait of the personalities behind the paint.
The series looks at the relationship between street art and graffiti, the interaction of global and local influences on the art found in each city, and how the rise of street art sales has led to an increase in paintings disappearing from the street - only to reappear on ebay.
Programme two follows Brazilian street artist Nunca as he travels to an exhibition at the Tate Modern in London.
"Nunca" - Never in Portuguese - is part of a wave of up and coming Brazilian street artists currently being invited to exhibit in Europe. Joby meets him in his Sao Paulo suburb.
Nunca started out aged 12, writing graffiti and pichação - the stark black lettering which is unique to Brazil. It adorns every surface below 15 feet in Sao Paulo.
Over the years his work has developed into a more pictorial style, incorporating indigenous art and in particular the use of cannibals.
Joby discovers how the scarcity of good quality spraypaint, and lack of information about the New York scene - led to Sao Paulo artists developing their own distinctive form of street art.
He goes out with Nunca to paint a news-stand in broad daylight and then, when he arrives in London, finds out if Nunca can transfer his counter-cultural message to the major museum space of the Tate Modern.
First broadcast 28 November 2008