Painter, filmmaker, photographer, philosopher, all-round celebrity, Andy Warhol is an outstanding cultural icon. He revolutionised art by bringing to it images from popular culture - such as the Campbell's soup can and Marilyn Monroe's face - while his studio, the Factory, where his free-spirited cast of 'superstars' mingled with the rich and famous, became the place of origin for every groundswell shaping American culture. In many ways he can be seen as the precursor to today's 'celebrity artists' such as Tracey Emin and Damian Hurst. But what of the man behind the white wig and dark glasses?