IMPRESSIONISM is the art movement that just refuses to go away. The proven box office draw of Degas, Manet, Renoir and the rest means there is always an audience for exhibitions of their paintings, which leaves curators and art gallery directors with the problem of finding new ways to recut and reinterpret what is essentially a finite body of work.

An exhibition looking specifically at Renoir's landscapes has just opened at the National Gallery in London and on April 1 a show examining the work of the impressionist colony which formed in Giverny opens in the French town's Musée d'Art Américain. Later, in May, the Metropolitan Museum of Art in New York hosts an exhibition of impressionist favourites but uses as the context for the show the sibling rivalry between the two men from whose collections the works are drawn - Robert Sterling Clark and his brother Stephen Carlton Clark, heirs to the Singer sewing machine millions.

On Saturday, London's Royal Academy of Arts opens the doors on an exhibition called The Unknown Monet: Pastels And Drawings. The exhibition aims to highlight a previously undiscovered aspect of Monet's career, namely his talent as a draughtsman. By pulling together around 80 works on paper - black chalk drawings, pastels, pencil sketches - The Unknown Monet shows that, while publicly the artist was dismissive of drawing, in fact he used it extensively throughout his career and in studies for even his most famous works.

MaryAnne Stevens, senior curator at the gallery, acted as consultant on the show, which tours to America later in the year. "Nobody has before addressed this particular aspect of Monet's creative output," she says. "Instead they've always focused on the paintings and the creation of the garden and so on, rather than thinking about what he did in preparation for the finished works."

The reasons lie in Monet's own self-mythologising: he gave many interviews over the course of a long life but tended not to mention his use of drawing. Sometimes he went as far as to disown its importance in the creative process.

"He led us astray, in a sense," says Stevens. "So as a result nobody has ever bothered to seek out the drawings because the assumption was that he put up his easel in front of his motif and painted directly in front of nature - and that was the assumption that Monet wanted us all to have. He wanted to be seen to be instinctive because that was all part of the impressionist ethos. In fact he received a quite conventional artistic training and drawing was an essential part of that."

In this, the painter had a modern grasp of what it is to be an art star. "Monet knew how to market himself," says Stevens. "He was a supreme manipulator and he knew very well that to lay great emphasis on actually painting directly on to the canvas, out of doors, in front of the motif was what people expected of him. Therefore he had to reinforce that notion rather than peel it away."

One of the most illuminating aspects of the exhibition is its display of Monet's caricatures, mostly executed around his native Le Havre. We tend not to think of Steve Bell and Gerald Scarfe as having any connection with the man who painted water lilies over and over again but they are heirs to a tradition he inhabited, and to which he brought just as much venom and mischief as they do today. Monet was inspired by Honore Daumier, a celebrated caricaturist praised by Baudelaire in 1857 in one of the first serious-minded pieces of criticism about what are essentially cartoons.

Dandy With A Cigar, a study in pencil and gouache made around 1857 when Monet was in his late teens, shows a red-nosed libertine puffing on a huge cigar. The Painter With A Pointed Hat is an equally comic sketch, showing a colourfully dressed artist holding a paintbox. His long hair spills from under a wide-brimmed hat and his handlebar moustache, enormous red bow tie and short cape make him look like a rakish Iberian troubadour. Others poke fun at real people rather than types: local dignitaries, mostly, who are represented in the pomp of office but with elongated necks, prominent noses, big heads, small bodies.

Monet died in December 1926 aged 86 and with his death came the start of the period which would see the real escalation in the value of his paintings. In 1927 however, the owner of a suite of Monet's early caricatures took them to his Paris dealer with a view to selling them and left empty-handed. It was only gradually, as Monet's auction house status grew, that his drawings began to acquire value.

But the exhibition doesn't deal just with Monet's formative years. Instead it shows that alongside the more famous paintings lies a parallel body of work. For instance the first exhibition by the group of artists who came to be known as the impressionists, held in the studio of a photographer on the Boulevard des Capucines between April and May 1874, featured five now-famous Monet canvasses. Among them was the painting which gave the group their name, the iconic impression, Sunrise, a view of Le Havre.

Also included, however, were seven untitled sketches in pastel, a fact which is often overlooked by critics. Indeed one of the innovations adopted by the impressionists in organising their own exhibition was the grouping of paintings with their associated works: sketches, studies, pastels. This hadn't been done before as the Salon, the annual state-sanctioned exhibition, showed them separately. Degas had actually written to the Salon organisers in 1870 complaining about the practice.

"These drawings are challenging because they're so unexpected," says Stevens. "Today nobody thinks to look at Monet as a pastelist. They look at him as a painter of poppy fields and boating scenes and views of the River Thames. But the idea he did commit to paper in pastel is a revelation."

Perhaps the most intriguing drawing in the show is the one for which art historians have neither a date nor a name. Described variously as being rendered in red chalk or charcoal it is titled Portrait Of A Woman.

As for the sitter, that too has been argued over. It may show Camille Doncíeux, Monet's first wife. It may show Alice Hoschede, his later companion. It may even show Hoschede's daughter, Suzanne, who modelled for Monet in the late 1880s and appears in 20 canvases. The only thing which dates the drawing is the style of hat, which places it in the early 1890s.

As for its meaning - it is dense, accomplished and intimate yet bears no relation to any painted work - that remains lost. Still, as the exhibition's American curators Richard Kendall and James A Ganz note in their catalogue introduction, the drawing is "emblematic of the uncertainty that has long surrounded Monet's graphic oeuvre". Moreover, they note, the drawing has never before been shown in an exhibition of Monet's work.

Is there a danger, though, that this exhibition will do exactly what Monet guarded against for much of his career: rob him and his canon of its mystique? No, says Stevens."I think it actually provides a much deeper understanding of how Monet functioned as an artist. You begin to understand how he uses his sketches. That shows the process of selection, that shows him thinking about what would make a good composition, a good painting. So it allows us to enter into his creative mind."

And that must be a journey worth making.

The Unknown Monet: Pastels And Drawings is at the Royal Academy of Art, London from March 17 to June 10