Lucian Freud at Tate Britain

Lucian Freud often paints the same people again and again. From the early - wide-eyed, startled-looking, slightly otherworldly - portraits of his first wife Kitty, to the majestic canvases of flamboyant performance artist Leigh Bowery some 40 years later, Freud has always searched for that ineffable quantity: the inner life. 'If you don't know them,' he once said of his sitters, 'they can be like a travel book.'

Freud's brilliance as a draughtsman is coupled with an amazing eye for detail: the paisley patterns on his elderly mother's dress are given as much attention as her stubborn, jowly face. Often accused of painting putrefying flesh, only occasionally does he seem to overdo it.

Eyes obscured by the fleshy folds of skin, the face of a broad-featured young woman appears to be collapsing under myriad patches of livid colour, breaking up its bland surface as though seen through a prism; the skin has become something almost grotesque. But it is a relatively early work, painted in 1959, and Freud is still searching for a more expressive language to convey the texture of living, breathing flesh.

Yet Freud's small-scale head and shoulder portraits are also among the most startling and powerful images in this major retrospective. His 1952 portrait of painter John Minton, who committed suicide five years later, shows its subject baring his teeth in a study of undeniable wretchedness.

His painting of the artist Frank Auerbach, executed in the mid-1970s, similarly captures something of the sitter's innate qualities: head tilted forward, so that his huge high forehead dominates the small canvas, it manages to convey something of Auerbach's cerebral intensity, revealing him as a deeply private individual.

Freud often gets his sitters to maintain uncomfortable poses for hours at a time, so his naked figures inevitably possess a clumsy, often touching vulnerability. He is attracted to painting the sturdy and the statuesque. Yet, with her head stretched back and her arms flung outward, there is also something deeply tender about his 1994 painting of 'big Sue', the benefits clerk.

The last two rooms contain Freud's most powerful work. Now in his eightieth year, he has just got better over time. Two monumental paintings of Leigh Bowery dominate the last decade, one in which he poses uncomfortably on the floor with legs raised awkwardly on to an adjacent bed, the other in which he sits bolt upright, his impressive frame squeezed into a tatty armchair. More recently, there are the two large canvases of current lover Emily Bearn, 51 years his junior.

The paint has coarsened considerably in these last few works - it's a world away from the thinly painted, smooth surfaces of the early 1950s. In the large-scale canvas of Bearn reclining, we see her worn, white, slipper-like shoes discarded on the floor; it's these incidentals which soften the unrelenting, coldly clinical gaze.

Finally, as he is still assiduously working on it, we have been told that Freud's eagerly anticipated portrait of the pregnant Kate Moss might yet appear by the end of the exhibition's three-month run. It's startling to think that this is an artist still improving over time, still, certainly, in command of his powers. Fisun G¸ner

Until Sept 22, Tate Britain , Millbank SW1, daily 10am to 5.40pm (last admission 5pm), £9, £6.50 concs. Tel: 020 7887 8008. www.tate.org.uk. Tube: Pimlico