Jacques Herzog and Pierre de Meuron have just completed the world's most iconic building in this decade of iconic buildings, the Olympic Stadium in Beijing, which won the inaugural Design of the Year for Architecture award from the Design Museum yesterday. They've built the world's most famous and successful art gallery, Tate Modern, and, aged 57, built dozens more, including their latest, Caixa Forum, in Madrid.

Apart from football, there is only one subject that raises their ire: Beijing. Or, more specifically, the decision of Steven Spielberg last month to pull out of directing the opening ceremony of the Olympic Games this summer, citing China's human-rights record and involvement in the Darfur tragedy in Sudan.

“It's very cheap and easy for architects and artists and film-makers to pull out or to make this kind of criticism,” Herzog says. “Everybody knows what happens in China. All work conditions in China are not what you'd desire. But you wear a pullover made in China. It's easy to criticise, being far away. I'm tempted almost to say the opposite...How great it was to work in China and how much I believe that doing the stadium [and] the process of opening will change radically, transform, the society. Engagement is the best way of moving in the right direction.”

“It would be arrogant not to engage,” de Meuron adds. “Otherwise no politicians could go there, no athletes. You would just close the borders.”

“Literally everybody in the Western world trades with China,” Herzog continues. “This is a fact. So why should an architect not?”

They hope that the biggest single symbol of the West's engagement in China - their stadium - might be, if not a Trojan horse, then a building that embodies and moves forward loose but, in China, radical ideas, such as freedom. The very architecture - an open basket or “bird's nest” of girders in which visitors can choose their own, random paths, is pointedly designed. “We wanted to do something not hierarchical, to make not a big gesture as you'd expect in a political system like that,” de Meuron says, “but [something that for] 100,000 people [is still] on a human scale, without being oppressive. It's about disorder and order, apparent disorder. It seems random, chaotic, but there's a very clear structural rationale.”

“The Chinese love to hang out in public spaces,” Herzog adds. “The main idea was to offer them a playground.” The Chinese Government, they say, has carried out their wishes to the letter. They make a distinction between creating a building that fosters a country's ideology - say, Albert Speer's work for Hitler - and one that seeks to transform it.

Herzog, the front man, the politician, with his shaven skull and sunken, unsmiling cheeks, looks as if he wouldn't suffer fools, though his gravitas is leavened occasionally with camp, gossipy asides. De Meuron, by contrast, doesn't like limelight. He looks like Beethoven: a bit tortured, introverted, with mad, wiry hair and a furrowed brow, as if he's the one always left to worry if they've ordered enough rivets for their latest stadium. They look as if they spend their evenings discussing Hegel, not American Idol. That they're rabid fans of FC Basel comes as a blessed relief.

In their Basel HQ they flit between desks, whispering together in corners, chuckling together at jokes like twins. “The chemistry works between us,” Herzog says. “We've never had problems of jealousy, or one being more visible. We do a better job together.” They've grown so close that it's hard to discern who does what. “No project is either Herzog's or de Meuron's,” he continues. “Maybe my strength is getting a project started, and Pierre then pushes it.”

“It's not only professional collaboration,” de Meuron adds. “It's friendship, and that's unique. To have no barriers in a relationship, whether it's husband and wife or a working collaboration, [means you can] be open to everything.”

They met at the age of 6 at kindergarten. “Jacques is more middle class, and I, as my name says, am more aristocratic,” de Meuron says. “But we grew up in the same way. If you grow up together you share the same secrets.”

“We had almost a professional relationship even as children,” Herzog says. “He was less interested in football, so instead of doing sport together, what we did was produce things together. We copied things we saw on the street.”

At university Herzog drifted first into biology, though de Meuron says he “always knew” he wanted to be an architect, despite his (and Herzog's) family having no architectural connections and his parents' suspicion that architecture was somehow flakey. “They wanted me to be an engineer, something real.” “We just sort of drifted together,” Herzog says, “like it was inevitable.”

They did so at a critical moment. By the mid-1970s, modernist architecture had become discredited. Architects were after new directions. At university the pair studied under the influential theorist Aldo Rossi, who proposed returning to the traditional qualities of buildings. They prefer the left-field views of conceptual artists - as common as trams in Basel. They've collaborated with, designed spaces for and absorbed the methods of artists from Thomas Ruff and Rosemarie Trockel to Michael Craig-Martin at Tate Modern and Ai Wei Wei in Beijing. “Artists are clearly more interesting than most architects,” Herzog continues. “They are obliged to invent their own language.”

Herzog and de Meuron's architecture is for all five senses - “a sensory whirlwind,” Herzog says, “otherwise we chop it off, we limit it, we mutilate it.” They owe this interest in materials to meeting at the start of their careers the artist who perhaps influenced them most, Joseph Beuys, who “made us aware of the invisible qualities of materials”, Herzog has said. Their every project is distinguished by celebrating the multisensory qualities of the stuff that makes up the skin and flesh of buildings - concrete stained by the rain, a thick fudge of pebbledash smeared over their Schaulager art gallery, Basel, the metal foil smothering the Walker Art Center, Milwaukee, the glass lenses wrapped around their Prada store in Tokyo.

In May they release new designs for their extension to Tate Modern. They first attracted criticism that their glass zigurrat would not fit in. “I don't agree,” Herzog says. “It will fit in. But we are still working on it. It will be very different. We have to respect that. The building will be very radical too. But if you come up and demonstrate your otherness with a huge beard or long hair or you tint your hair green - who's interested in that in architecture?”

De Meuron says it “is critical not to build all over the world in the same way”. Unlike their peers, Frank Gehry, Daniel Libeskind, Norman Foster and Zaha Hadid, they have no signature style. Every building they make is unlike the last, united only by their odd looks and uncanny mood. In an age of Identikit skylines, you can be sure of getting something different when you hire Herzog & de Meuron, or, more accurately, when they hire you.

Jacques Herzog is speaking at Tate Modern on April 8

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