Saturday evening at Sonar and the place is jumping. Julee Cruise, the Twin Peaks diva, is crooning in a basement with the New York mavericks Khan. The Russian DJ Vadim is mixing up a storm for the crowds grooving on the AstroTurf in front of the main stage. People are still tripping their way through the Invisible London multimedia exhibition. The German Kitty-Yo label is showing its wares to people who have well and truly abandoned their candy-striped deckchairs. Only 12 hours to go!

Every year since 1994, Sonar - otherwise known as the Barcelona International Festival of Advanced Music and Multimedia Art - has brought hundreds and, lately, thousands of acts and delegates to the Catalonian city. Over three days and nights, new names are showcased, established artists celebrated and numerous chins scratched, especially when it comes to deciphering the event's unique branding and visuals.

While many other festivals aim for a similar crowd, only Sonar consistently attracts the cream of the international underground. It could be because of its reputation - an eclectic booking policy ensures Sonar is ahead of the curve when it comes to cutting-edge electronica - and location, as Barcelona in June is hard to beat, but Sonar has a far higher quota of "early adopters" and connected folk than its European equivalents put together.

Although it shares equal billing with multimedia, music is what makes Sonar tick. The festival concentrates on electronic acts of every size and shape. By day, four stages at the Centre de Cultura Contemporania de Barcelona, in the middle of the city, host live acts and DJs. After sunset, the action switches to Montjuic 2, an impersonal convention centre in the western suburb of L'Hospitalet.

This year's festival, held last weekend, saw a growing number of fringe attractions, with many delegates forgoing the trek to the night-time events in favour of checking out clubs around the Ramblas, but the day stages are certainly diverse.

An early highlight was a solo performance by the avant-garde composer Terry Riley. A pioneering minimalist, the Californian displayed some characteristically mischievous and playful touches in his loops and melodies. A short sample from Salome Dances For Peace, his epic five-quartet cycle from 1989, demonstrates that any current adventures in electronic ambience were predated by Riley and his peers.

It is a fact Kieran Hebden would acknowledge. Along with the Canadian Dan Snaith, from Manitoba, the 23-year-old, who fronts the highly lauded outfits Four Tet and Fridge, has been responsible for the emergence of a warmer brand of electronica.

Given that his name is one of the most dropped of all, his deejaying set alongside Laurence Bell, the head of Domino Records, was understandably well attended. Clues about future directions were hard to discern, although he lobbed in some Four Tet rarities.

Anyone favouring a more folky side to their electronica would have been enthralled by Lemon Jelly. With only a handful of EPs to their name, Fred Deakin and Nick Franglen have mastered an intoxicating swing that smartly balances laid-back beats and acoustic meanderings. Bookended by the maverick selections of Andy Votel and the pockmarked sound of Capitol K, this was a slot to baffle anyone keen on pigeon-holes.

Other treats for day trippers included the sunny drum and bass of the Brazilian producer Patife, the sultry fried soul of Nicole Willis and the intriguing new work of Irmin Schmidt, the Can keyboardist.

In the SonarLab tent, labels got a chance to impress, with the likes of Steve Shelley, the Sonic Youth drummer, and Jim O'Rourke, the innovative American, showing off their Smells Like and Moikai imprints.

John Peel, the veteran BBC Radio 1 DJ, John Peel received a suitably respectful reception for his Peel Sessions, although Charles Webster's beautiful selection of deep-house delights from the Statra label had far more dancers on the floor.

Those who forsook the thrills of city-centre clubs and cafes for the industrial landscape of L'Hospitalet were amply rewarded, especially on the opening night.

Sonic Youth, Shelley's seminal New York act, based their set around their experimental album Goodbye 20th Century, while the Icelandic pioneers Sigur Ros offered another welcome immersion into their epic world.

Both bands set a pace, but it was Zero 7 who had heads nodding. In only their second live show, the cult British act revealed new angles on Simple Things, their recently released debut album, revelling in the chance to improvise and seek new directions.

Anyone looking for a name to watch would have been impressed by The Youngsters. A duo from Montpellier now signed to F Communications, the label of the DJ Laurent Garnier, their incendiary brew of rough techno and hardcore breaks may lend itself to comparisons with Daft Punk or the Chemical Brothers, but the energy here is of a fresher, more vibrant hue.

While the night's galaxy of superstar DJs, from Carl Cox and Darren Emerson to Masters at Work and Jeff Mills, kept many out until the wee hours, most ventured at some stage to check the multimedia side of Sonar.

Even though they are underexposed exhibitions give underground talents a space to shine. After previous focuses on Berlin and Barcelona, this year's exhibition headed to London, and in particular the city's unseen cultural underground. Invisible London attempted to find the threads that link cultural activities there, from music to graphic design.

If some productions worked - Peter Cusack's Your Favourite London Sound produced a diverse urban soundtrack - some never left the starting blocks, the premise of the London Sampler video installation proving untenable, for example.

The exhibition's strengths came to the fore when it focused on musical themes. Olly Hewet's Disseminating Music contained finely tuned snapshots of London's underground music industry, examining retail outlets and distributors and their place in the greater scheme.

A showcase of graphic designers highlighted some brilliant work, in particular Trevor Jackson's idiosyncratic and arresting work for the Output label.

In comparison with the exhibitions of previous years, however, Invisible London was weak and unimaginative, displaying none of the excitement or verve of previous installations.

There is a sense of tokenism to the multimedia side of the festival, and the curators did not help their cause.

As the festival wound down, however, the organisers were celebrating another successful year. Despite the counter-attraction of fringe events, such as the Leaf label's showcase on the Friday night and other alternative Sonar events, numbers were up - more than 40,000 at Sonar By Night - and the momentum was maintained.

Don't be surprised, though, if Sonar 2002 brings its night-time activities back into the city. And you can take it for granted that next year's bill will contain as much innovation and experimentation as ever.

Further information at www.sonar.es