A comprehensive exhibition of Expressionism is on display in Venice. Andrew Lambirth is taken from brutal introspection to colourful optimism.

The Expressionists tried to create images that conveyed states of mind. They went all-out for an emotional impact, achieved by way of exaggeration and distortion. In 1905 a group calling itself Die Brucke, or The Bridge (to the future), was formed by Kirchner, Heckel and Schmidt-Rottluff, amongst others. This was the real beginning of Expressionism. Another sub-group calling itself Die Blaue Reiter, or The Blue Rider, came into existence in 1911 under the leadership of Kandinsky and Marc. They painted from "compelling inner necessity" and hoped for a harmony between life and art. Before the First World War shattered so many illusions, they strove for Utopia, although, with hindsight, the Expressionist goal seems more like its opposite - an angry hell of festering selfhood, of suffering and brutalisation. Going inward in search of the soul was a dangerous journey. Several artists, like Kirchner, suffered mental breakdown en route.

In Venice now, at the Palazzo Grassi, is a remarkable show of German Expressionism. The Palazzo Grassi was bought by Fiat in 1984 and lavishly restored. A series of seminal exhibitions has since been staged there all emanating from Fiat's impressive role in cultural sponsorship.

"German Expressionism" starts out with a room of troubled and melancholy self-portraits. The very first you see is by Otto Dix, a harsh depiction of himself as a soldier. Here the long dark night of the Northern self finds expression; how very far from the sunny rationalities of Mediterranean classicism. But there had to be some relief from introspection, and many of these artists went off to paint and frolic on Baltic beaches. Otto Muller, Max Pechstein and Kirchner all did successful alfresco nudes.

One of the most powerful must be by Karl Schmidt-Rottluff, called Three Nudes (Picture of Dunes by Nidden). The simplicity of the composition and the solidity of the figure group recall the influence of primitive sculpture, and there is a debt of colour and rawness to the Fauves, though taken much further. This is the up-beat optimistic side of Expressionism.

The star of the show has to be Ernst Ludwig Kirchner. There is a substantial body of work by him, not merely the jagged-edged city-street masterpieces of his early years, but a couple of the late brightly coloured landscapes as well. Rarely has he been shown to better advantage. Kirchner was deeply drawn to the Gothic, as much to its sharply pointed drama as to its mystical directness - look at the soaring verticality of his tremendous Bridge on the Rhine in Cologne. Kirchner wanted to shock the viewer into an unexpected response by playing up the ugliness of the modern city and its inhabitants. He adopted a method of painting in which hacking directional brushstrokes imitate a woodcarver's marks. Five Women on the Street is deliberately intimidating: there is a sharp edge of danger beneath the flaunting and display.

The Kokoschka room is important. His painting The Hunt reappears as a great source for so much later German art - witness the work of Georg Baselitz. There are many other treats: the Fauve-like gentle Jawlensky; the massive pastoral calm of Franz Marc, here instanced in an idyll of shepherds. In a couple of beautiful paintings by Kandinsky of rich deliquescing colours, the real world is melted away in favour of a more perfect spiritual existence. By contrast, the emotional and literal darkness of Beckmann, its sculptural unavoidability, is still harrowing. In The Corner House Ludwig Meidner paints a crazy, collapsing building of mad angles, like something out of The Cabinet of Doctor Caligari.

There is a whole slew of little-known names, by and large artists of little talent. They serve to throw the better work into sharp relief, but must exhibitions seek to be all-inclusive in this way? It proves that the organisers have done their research, but I'm not convinced that Heinrich Ehmsen and Walter Jacob, among others, contribute much. Otherwise, the exhibition is well-paced, elegantly and spaciously hung.

It is also themed, divided into subjects such as "Hell and Paradise, Man and the Metropolis". This is more of a distraction than a help. Curators should not be encouraged to force an argument on to art. Paintings are much more than examples to illustrate a theory. What is useful, however, is the audio-visual room. Snatches of music by composers as various as Berg, Bartok, Reger, Webern and Hindemith echo through the show. A sophisticated slide show projects images of, for example, Kirchner's friends and atelier, interspersed with archive film snippets. Photographs are matched to paintings as their source. Altogether, this audio-visual interlude provides a compact introduction to the show, encouraging a more convincing "reading" of the exhibition than the rather tenuous theming. Highly recommended.

'German Expressionism: Art and Society': Palazzo Grassi, San Samuele 3231, Venice, to 11 Jan, open daily 10am-7pm