The Centre for Alternative Technology (CAT) is housed in a former quarry just outside Machynlleth, Powys. It was founded in the early 1970s as a place to develop technologies to solve the planet’s problems, particularly the dependence on fossil fuels. Today, with climate change a global reality, these once marginal concerns are at the heart of government policy and CAT has become a centre where students and professionals can develop their skills in renewable energy and sustainable building, either as an MSc or as a vocational course. The buildings, which have been developed at CAT over the last 40 years, illustrate a miniature history of green building, demonstrating changing priorities and possibilities. The latest building to be completed on the site, the Wales Institute for Sustainable Education (WISE), combines exceptional environmental performance with a lucid modern aesthetic. It is an exemplar of all that has been learned about sustainable construction. The WISE building is, in effect, a miniature campus, tucked into the folds of the quarry site. It provides the setting for courses in tertiary education and conferences on environmental subjects. The accommodation includes a restaurant, bar, foyer and 200- seat lecture theatre, offices, seminar rooms and other teaching spaces, and 24 double bedrooms with en suite facilities. All these spaces are arranged around courtyards and terraces so that every room enjoys daylight, natural ventilation and a fine view out over this beautiful site. The building is on a larger scale than other buildings at CAT and has meant a departure from the usual approach, which is generally based on craft techniques and self-build; the architects, David Lea and Pat Borer, have designed it to be more amenable to mainstream construction, albeit in it a modern and affordable form. The architectural concept is an evolution of all the buildings at the CAT site; it takes the best of what has been done before in environmental building, but on a much larger scale. When students come to WISE to learn about green building techniques the building itself will be used as an example of the best possible practice. It is constructed of materials with low embodied energy: glulam timber frame, hemplime walls, rammed earth, lime renders, slate, cork, home grown timber floors and finishes of natural paints and stains. It is highly insulated and airtight, with heat recovery to some areas.