H & DM's de Young maturity.

While modest in its architectural expression, the completion of Tate Modern was a significant moment in the evolution of the now globally recognised design lab, Herzog and de Meuron. Bringing the Swiss architects to a British audience for the first time since an influential show at the 9H Gallery in the 1980s, this appointment led to other commissions, including their Stirling Prize winning Laban Dance Centre in 2003. Five years after its completion they have returned to Bankside to reapply their considered invention to Giles Gilbert Scott's awesome art/powerhouse. With design work continuing on a sizable expansion project south of the Turbine Hall, completing their original competition-winning strategy to extend pedestrian links across the level one bridge, they have also been invited by Nicholas Serota to produce Tate Modern's first major architecture exhibition.

Seen originally in their home town of Basel last summer, No 250--An Exhibition was designed to give insight into their working practices. With no web site, an extremely tight door policy (even employees' partners are screened before visiting their loved ones at work), and a cool phone line that is rarely answered, this is a rare opportunity. As at Schaulager in 2004, the exhibition's main attraction focuses on a jumble sale of 'waste'; unfortunately, however, nothing is for sale, and rummaging--however tempting--is certainly not encouraged. Originally entitled Waste and Sweet Dreams, more than 70 projects are represented on simple tables that are loosely scattered (or rather carefully placed) in the Turbine Hall's lofty east end. The by-products of their highly experimental design processes are fascinating, although perhaps more appealing to designers and artists who use similarly archaic techniques. To a wider public audience more accustomed to reading the cool and slick imagery of commercial architecture, this display may come as something of a surprise, which is no bad thing. The exhibition serves as an ideal opportunity for the Tate to promote the crafted, creative, and dare I say sculptural qualities of architecture. With full-size prototypes, mock-ups and models made with sample materials, tactility, texture, tone and technique are expressed. And, while there has been criticism of their work being increasingly concerned with surface and image, models also demonstrate Herzog and de Meuron's commitment to the form of space as well as the form of matter. If time is taken to trace each project's evolution, evidence emerges of H & dM's strategy to slow down the process of form finding. Instead of rushing to anticipate a building's silhouette, their hands-on workshop processes complement more theoretical ideas of place making, allowing context and detail to converge, and forms to evolve gradually. As a recent visit to San Francisco's de Young Museum underlined, their working method remains consistent throughout with no great leap required between design and construction. As passionately demonstrated on site by architect Jayne Barlow, even during construction the H & dM team, supported by excellent contractors and craftsmen, continue to inspire by producing highly inventive, beautifully crafted, work-of-art buildings.

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Herzog & de Meuron. An Exhibition, Tate Modern, London until 29 August. Site photographs by Rob Gregory.