From Picasso's Cubism and Duchamp's readymades to Warhol's silkscreens and Smithson's earthworks, the art of the twentieth century broke completely with earlier artistic traditions. A basic change in the market for advanced art produced a heightened demand for innovation, and young conceptual innovators - from Picasso and Duchamp to Rauschenberg and Warhol to Cindy Sherman and Damien Hirst - responded not only by creating dozens of new forms of art, but also by behaving in ways that would have been incomprehensible to their predecessors.

When it comes to art, London is best known for its galleries, not its graffiti. However, not if photographer Martin Bull has anything to say about it. While newspapers and magazines the world over send their critics to review the latest Damien Hirst show at the Tate Modern, Bull, in turn, is out taking photos of the latest street installations by guerilla art icon Banksy.