It is usual to wait a few years before calling a work of art "great". To call a new piece by the term you would use for, say, a painting by Gustave Courbet, surely devalues its currency. But I can't think of any other word to describe Jeff Wall's picture War Game.

It is a big black and white photograph of boys playing on a sinister stretch of wasteland on the edge of what could be virtually any city in the world. A kid in the foreground sits amid a pile of waste objects, looking down. You don't immediately notice what he is looking at - a row of three human bodies, half-hidden by the scrap enclosure. The shock is complicated when you notice the grin on one "dead" soldier's face.

The date of the work is 2007, the pertinence to our time immediate. This is just a scene of boys playing, but there is a spectre of the violent and cruel in their game, as if the reality of 21st- century war has leaked into childhood's imaginary world. It is also an allegory of how easily we ignore horror. Across the wasteland, a woman walks by unseeing - yet the poverty of the scene, of the lives you picture for these boys, is horror in itself.

I have described this as a "big" photograph and I don't just mean the size of the print. It is big in ideas. Wall makes history paintings with a camera. He is not producing reportage. His staged and posed, aesthetically contrived photographs are not trying to get at the passing surface of events, but at their secret structure. Go to see it, and tell me he has not imagined the truth.

· Until January 19. Box office: 020-7930 5373.