The Italian Arte Povera movement, which burst on the scene in the early 1960s and lasted about a decade, was a manifestation of the unprecedented upheavals that swept through Western art at the time, generating a bewildering range of boldly alternative strategies, from land art to performance to conceptualism. In that, it was part of an important trend in contemporary art, but one that also, in the extent of its euphoric ambition, subsequently did something of a disappearing act.

Hence Zero To Infinity, an enormous exhibition at Tate Modern in London, is not only a celebration of the heyday of Arte Povera but also an exercise in rediscovery. And as you negotiate much of its vast expanse of gallery space, it feels like an archaeological expedition, because, despite optimistic preambles about how fresh and current it all is, a lot of the work has already ossified into the skeletal residue of its stylistic era.

Not that Arte Povera and its equivalents haven't been influential. They recognisably influenced the conceptually oriented work of the 1990s. But in, for example, the form of the Brit Art phenomenon, it was as if the once brave revolutionary ideas were being recycled in a trivialised, ironic way and, more to the point, shorn of their central, antimarket elements, dumbed down and shamelessly commodified.

Another enduring legacy is the Povera penchant for huge, theatrical gestures, perennially evident on the international art circuit in the form of ever grander, more bombastic and - alas - ever emptier installations. It sometimes seems the greater the logistical effort, the slighter the artistic pay-off.

The critic Germano Celant coined the term Arte Povera. He intended it to encompass the activities of like-minded artists on a global scale, but it tends to be associated with a specifically Italian context.

It came to be seen as referring to the artists' use of "poor", usually natural materials, such as earth, water and stone, as opposed to conventional, prestigious or expensive fine art materials, such as bronze and oil paint.

But Celant originally posited this new poor art as being radically opposed to conventional "rich" art, with its baggage of technique, iconography, artifice and ambiguity. By contrast, he wrote, Arte Povera artists broke through the barriers between object and idea, artist and artwork, artwork and spectator, offering engagement with the real. So they could, and did, use whatever materials they liked. What counted was how they used them.

There is something innocently Utopian about Celant's hyperbolic enthusiasm for the notion of these artists starting from scratch, from what Roland Barthes famously termed "degree zero". They were operating in an economically hectic environment, through a decade in which political awareness and involvement developed rapidly. They reacted to the dual strategies of abstract and figurative post-war art, but of course there was no degree zero.

They inevitably adapted aspects of what they reacted to, and they had precursors, notably the anarchic, tragically short-lived Piero Manzoni, who signed naked bodies and christened them "living sculptures". He also signed cans of Merda D'Artista, presumably filled with what it says on the tin, though it's unlikely that anyone has opened one yet to find out.

The Tate show allows generous space to 14 artists (only one of whom is a woman). For most of them, the years covered by the exhibition - 1962 to 1972 - were their glory days. While a few, including Jannis Kounellis, Mario Merz and one of the younger artists, Giuseppe Penone (the subject of a Douglas Hyde Gallery show a while back), are still very well known, chances are that for most visitors to the Tate, this show will be their introduction to the majority of the artists.

There is no unanimity of approach in their work, some of which has lasted well, some less so. Penone's startling photographic self-portrait, in which he wears mirrored contact lenses, is strong enough to form the exhibition poster, and his work generally, particularly that consisting of subtle interventions in natural processes, has a renewed relevance.

The same cannot be said for Piero Gilardi, who presents polystyrene slices of reality in the form of fragments of ground, from mountainous terrain to stream beds, but not half as well as the Boyle Family, who, with various conceptual embellishments, have made this sort of thing their lives' work.

Similarly Giulio Paolini's ploddingly pedantic deconstruction of painting, enacted over umpteen pieces, all delivered with the comfortable smugness of being the cleverest boy in the class. They take us close to Brit Art territory, and it's not that they're without interest, but they are slight.

There always seemed a brittle quality to the comparatively well known, memorably named Michelangelo Pistoletto who, like Paolini, invested a lot of thought and effort in looking at how we look at art.

These works come close to pop in their stylistic slickness, but then there is a surprise in the form of a couple of beautiful sculptures by Pistoletto, including a globe made from newspapers and a Lightbulb Curtain: shades of Felix Gonzales-Torres and any number of other artists since Pistoletto made these pieces, between 1966 and 1968.

The work of Giovanni Anselmo has a comparable elegance and clarity. One of the most talked-about pieces in the show is his sculpture consisting of a block of stone bound to a larger block with a head of lettuce forming a cushion between.

It looks terrific, and the idea is that, as the lettuce rots, the binding wire will lose its purchase on the stone and it will fall to the floor. Precariously suspended chunks of stone (precarious-looking, that is, but presumably not actually precarious) are Anselmo's calling card.

Among his other pieces, Entering The Work, a photograph picturing him running away from us into an expanse of bare hillside, is as crisp and engaging as Penone's self-portrait.

It would be easier to be enthusiastic about the sculptural installations of Kounellis - he is Greek by birth but on record as considering himself an Italian artist - if he didn't seem quite so responsible for encouraging vast amounts of comparable but vacuous efforts by others.

Often, his forceful, pertinent use of materials suggests a correspondence with Joseph Beuys, but by comparison he lacks Beuys's uncanny and perhaps morbid feeling for the physical properties of materials.

The one woman who featured in the original Povera line-up, Marisa Merz, shows a diverse group of punchy sculptural pieces that have in common a questing, inventive use of materials and a direct connection to bodily experience.

Pino Pascali, who died in 1968, was the one with a taste for light artillery, hence the compact field gun parked in the middle of the Tate. He was photographed, in military uniform, astride a missile like the crazed pilot in Stanley Kubrick's film Dr. Strangelove.

Luciano Fabro's explorations of the dimensions of gallery space, including a lot of messing around with folded sheets and measuring rods, look laboured now, though his tender little sculpture The Baby is surprisingly modest and effective.

Mario Merz started making igloos from a huge variety of materials in the 1960s, and they gradually became his trademark. Commentators have seen a connection between his igloos and Tracey Emin's tent, one difference being the former's political engagement as opposed to the latter's personal history. Another might be Italian stylishness against Emin's hotchpotch needlework. But her brooding introspection seems every bit as legitimate as the tenuous political associations of Merz's igloo, embellished with its motto by Vo Nguyen Giap, the North Vietnamese general.

Zero To Infinity is a generous show in allowing so much room to an important group of artists whose role in shaping contemporary art, though generally acknowledged, is probably underappreciated. Not everything stands up to scrutiny 40 years on, but a surprising amount does - and is likely to have a significant impact on younger artists and art students.

Zero To Infinity: Arte Povera 1962-1972 is at Tate Modern (+44 20 7887 8000), London, until August 19th