'A story should have a beginning, a middle, and an end... but not necessarily in that order.' - Jean-Luc Godard

A balcony at night. A man sharpens a razor blade. He observes a small cloud moving towards the full moon. The head of a girl comes into view, her eyes wide open. The cloud moves across the moon. The razor blade slices open the girl's eye.

Thus begins Un Chien Andalou (An Andalusian Dog) and the film career of Luis Buñuel. It is one of the most startling images in any film, and still retains the power to shock, over 70 years later. Made under the influence of André Breton's Manifesto of Surrealism (1924), and co-conceived with the artist Salvador Dali, its series of unconnected incidents was intended to follow the logic of a dream: ants emerge from the palm of a disembodied hand (an archetypal Daliesque image); priests are pulled along the ground; dead donkeys lie on two pianos. Although the film defies logical explanation, its rich supply of images from the unconscious can be read as a study of repressed sexual impulses.

Un Chien Andalou - the title is unrelated to anything in the film - was one of many avant-garde classics of the first part of the 20th century, most of them drawing from the other arts but retaining the specificity of cinema. There remained a strong experimental streak in films running parallel to the commercial cinema until the late 1960s, after which there was a sharp decline.

A few decades ago, the names of Luis Buñuel, Robert Bresson, Carl Dreyer, F W Murnau, Sergei M Eisenstein, Dziga Vertov, Abel Gance, Jean Renoir, Jean Vigo, Federico Fellini, Alain Resnais and Jean-Luc Godard, who worked in relatively "mainstream" cinema, could stand beside such diverse icons of modernism as Pablo Picasso, Marcel Duchamp, Arnold Schoenberg, Walter Gropius, André Breton, James Joyce, T S Eliot, Piet Mondrian, Jackson Pollock, Oscar Niemeyer, Bertolt Brecht, Karl-Heinz Stockhausen and Samuel Beckett.

It should be remembered that the birth and growth of cinema was almost immediately parallel to the birth and growth of modernism in the other arts. Film is generally at its best when it recognises its roots in modernism, i.e. when it rejects conventional notions of realism, disengages from bourgeois values, and questions the primacy of narration.

From the beginning of cinema, film artists working in the new medium understood that its strength was not in straight narrative, something literature or the theatre could do better. While commercial cinema, especially Hollywood, continued with the conventions of 19th-century literature and theatre by producing illustrated novels and "opened out" plays, modernists looked towards non-narrative film form, or considered narrative as secondary to style. They disturbed the accepted continuity of chronological development and attempted new ways of tracing the flow of characters' thoughts, replaced logical exposition with collages of fragmentary images, complex allusions and multiple points of view. They resisted the commercial film in favour of "art cinema", to equal the other arts in seriousness and depth. As early as 1918, the French poet Louis Aragon wrote that "cinema must have a place in the avant-garde's preoccupations... if one wants to bring some purity to the art of movement and light".

Riccioto Canudo, the Italian-born French critic, argued in 1926 that cinema must go beyond realism and express the film-maker's emotions as well as characters' psychology and even their unconscious. The formalist possibilities of cinema were expounded by the French "impressionist" film-makers and theorists Louis Delluc and Jean Epstein, and underlined by the montage theory put into practice by the great Russian film-makers in the 1920s.

Sergei Eisenstein, who wanted to film James Joyce's Ulysses, theorised and demonstrated his "montage of attractions", one of collision, conflict and contrast, with the emphasis on a dynamic juxtaposition of individual shots that forces the audience to come to conclusions about the interplay of images while they are also emotionally and psychologically affected. Eisenstein, whose dynamic montage reached its apogee with October (1928), believed, quoting Karl Marx, that "the bourgeoisie created the world in its own image. Comrades, we must destroy that image", a credo that Jean-Luc Godard echoed and practised 40 years later.

From the early days of the cinema, primarily in Europe, artists working in other forms were happy to contribute to the new art. Film, which Eisenstein envisioned as a "synthesis of all the arts", had become the new gesamtkunstwerk. The theatre guru Antonin Artaud, who appeared in Gance's Napoléon (1927) and in Dreyer's La Passion de Jeanne d'Arc (1928), wrote the screenplay for Germaine Dulac's La Coquille et le clergyman (The Seashell and the Clergyman, 1928). Now considered the first surrealist film, it was refused a certificate by the British censor because "if the film has a meaning, it is doubtless objectionable".

In 1924, René Clair's Entr'acte, an exercise in pure cinema, was conceived by the Dada painter and poet Francis Picabia, who appeared in the film alongside Man Ray, Marcel Achard, Erik Satie and Marcel Duchamp. In the same year, Marcel L'Herbier's L'Inhumaine was a veritable catalogue of contemporary French art. Each set was created by a different designer - Fernand Léger, Claude Autant-Lara, Alberto Cavalcanti, Robert Mallet-Stevens - and the film featured music by Darius Milhaud and dancers from the Ballets Suédois. Jean Cocteau was 41 when he made his first film (Le Sang d'un poète, 1930) and already famous as a poet, novelist, playwright, designer, painter, stage director and ballet producer, having fulfilled his promise to Serge Diaghilev, who had challenged him: "Etonne-moi!"