Forms, follies, functions.

While London's Tate Modern has the Unilever series, the Serpentine Gallery in Hyde Park has its pavilion, taking place on the summer calendar alongside Glastonbury and Wimbledon. This year, Portuguese architects (and friends) Alvaro Siza and Eduardo Souto de Moura collaborated with Arup's Cecil Balmond to create the Serpentine's fifth annual pavilion.

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Serpentine Director, Julia Peyton-Jones, is clear in her mind about what the Serpentine Pavilion has become. Albeit temporary in nature, the event surrounding each annual installation has become a permanent fixture on any art-goers calendar. It is not, however, an exhibition, she states categorically. Its costs are not covered by any curatorial budget, neither is it consciously seen as a series of works, chosen to relate each to the other, one from the next. It is instead a series of unique built spaces. It is as simple as that. It is furthermore specifically Architecture, not Art. Although seen by some as a series of sculptures in the park, each noted primarily for its formal qualities, artists have never been invited to design a pavilion, and they never will be. Although a number have expressed an interest, particularly those whose art blurs the boundaries between sculptural and structural speculation, Peyton-Jones is clear in her vision that the Serpentine Pavilion is about promoting contemporary architecture. It is also about introducing previously unseen work, with architects only invited if they are yet to build in the UK. As a publicly funded gallery, it is in this mission that the Serpentine's Pavilion achieves the most, bringing to a wide audience--knowledgeable gallery friends, casual dog-walking passers-by, architecture students, whoever--the opportunity to experience a space created by a previously remote architect. Fulfilling the challenge to demystify, educate and inform, the Serpentine Pavilions fulfil the role of the contemporary gallery with apparently effortless potency. However, to state the obvious, nothing is ever achieved without serious hard work, and here the process surrounding the realisation of each pavilion is as complex as any intricate construction project. As tenants of the Royal Parks, planning consent is one of the many hurdles, alongside the thankless task of seeking sponsorship, the challenge of working with overseas designers, and of course the small matter of producing a unique piece of cutting edge design in just six months (from commission to completion). In this process Peyton-Jones' commitment and enthusiasm (doubtlessly channelled through her feverishly busy curatorial team) has been highly acclaimed, noted earlier this year with her invitation to give the RIBA Trust Annual Lecture at Portland Place.

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The series began in 2000 with Zaha Hadid, two years after the completion of John Miller's gallery refurbishment, and coincident with the Serpentine's 30th anniversary. Originally envisaged as a three-day event, Hadid's design proved so popular with Chris Smith, then Secretary of State for Culture, Media and Sport, that he granted an immediate extension of time. With this, the annual summer pavilion tradition was born, with Daniel Libeskind following in 2001, Toyo Ito in 2002, and Oscar Niemeyer in 2003. This year, following the fallow season of MVRDV (who continue to cultivate what some consider to be their mountain out of a molehill vision) the invitation went to Siza and Souto de Moura, no strangers to the world's architectural community.

When choosing an architect, Peyton-Jones and her team of advisors make no attempt to second guess who may or may not make a popular choice, despite their commitment to bringing architecture to a wider popular audience. Instead, their ambition is that their choice will have a natural resonance with (and relevance to) British architectural culture. With hindsight all previous pavilions have struck a chord, with the exception perhaps of Niemeyer, who being well into his nineties was unable to participate in the process as actively as most, did not visit the site or the completed pavilion, and whose preoccupation to encapsulate his entire 70 year oeuvre in one single object resulted in a charming yet compromised structure; a graceful vision of concrete fluidity, uncomfortably clad in steel. Hadid, although remaining unbuilt in the UK, recovered well from the Gardiff Bay operatic farce of the late 1990s with her [pounds sterling]100 000 triangulated tent, and has since then gone on to build extensively around the world; Libeskind's inspired 18 turns, even in our memory, continues to outshine his disappointing homage to the constellation of Orion (built in 2004 on the Holloway Road); and Ito, although rumoured not to have been 100 per cent happy with his final pavilion, has gone on to build upon some of its inherent themes, most notably with his recently completed Tods store in Tokyo.

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Cecil Balmond, deputy chairman at Arup, has been closer to the process than any other designer. Since being invited to collaborate on the 2001 design, when Libeskind called him to say, 'let's show London what they can hope to expect from the Spiral' (Libeskind's recently shelved V & A extension building), Balmond and his team have been involved in three of the five pavilions. With his emerging media prominence, some critics have speculated that the series is in danger of becoming the Cecil Balmond show, without perhaps giving due consideration to the practical reality of how Arup involvement brings a necessary continuity. When working with remote international architects, Arup involvement helps to ensure deliverability for a client who has no contingency, either in the bank or the diary, and their ability to make manifest the vision of others will in years to come make the collective contribution of this series even more potent.

Balmond was involved again this year with Siza and Souto de Moura, and having collaborated before, most notably at the Lisbon Expo in 1998, the trio were no strangers to each others' working methods. In a process that concludes with a concept, rather than, as Souto de Moura alludes to, allowing a weak concept to be an alibi for a weak solution (this pavilion is no armadillo, despite the adopted metaphor), Siza recalls the to and fro process of Lisbon--Siza: 'Let's make a sling'. Balmond: '... and clad it with canvas'. Siza: 'I want it more solid. Concrete'. Then Balmond returned to London to make it work. Likewise, in this year's collaboration, the three-way toing and froing has produced a pavilion that bears unmistakable traits of each of the three designers, with Balmond notably yielding where necessary to Siza and Souto de Moura's lead. Perhaps because of this process, reflecting the preoccupations of each individual, this year's pavilion is the most contextual (Siza?), the most overtly crafted (Souto de Moura?), and the most structurally expressive of all (Balmond?). Perhaps such literal divisions are over simplistic, or not even important. What is important, however, is that this pavilion has brought--for a few delightful moments--something special to London. For the city's architectural community, the opportunity to meet Siza and Souto de Moura was a rare treat, as the pavilion's bulging form was full to bursting with students, leading architects and critics on the opening night. Hearing Siza admit that he 'didn't have much of an imagination', and how he needed 'a tree, a house and people' (ie, the Serpentine context) to inspire his design, was refreshingly insightful. As was the discussion that followed.

In very practical ways, the experience of the pavilion has much to teach, making a tangible response to many current architectural dilemmas, including the search for form and the problem of figure; the appropriate reinterpretation of traditional techniques; and the integration of emerging technologies. Here a standardised component (a 500mm deep, 39mm thick KERTO-Q spruce ply beam by MERK Finnforest), machine-cut in two planes, takes its unique and specific place in the gently curving puzzle. The apparently simple grid-shell structure has many subtleties; a roof that rises between two trees; a facade that twists in plane toward the vertical in acknowledgement of the Serpentine's symmetrical dominance; a translucent skin that lifts to give seated occupants clear garden views, and perhaps most ingenious of all (to those of a technical persuasion), Balmond's shuffle grid, where alternate structural members sit either side of the node points to achieve staggered curves, giving the illusion of a random, illogical, haphazard, and visually unstable structure. The more you look, the more is revealed. It even has a moment of spiky animal magic, with acrylic ventilation chimneys that contain individual night vision solar lamps.

Seeing how master-builders cope with such a pure brief, in a quest to tackle the proposition of the primitive shelter, is as inspirational as any major building project. If you can't visit Oporto, or find the mega scale of Balmond's global projects disengaging, then come down to the park. Here you will see the outcome of an architectural experiment that has tested how architects can respond to foreign contexts, and still be themselves, in a process that, to adapt a term used by Hans Ulrich Obrist, could be seen to be taking us towards an architecture of Glocalization.