The Architekturzentrum Wien (Vienna Architecture Centre) collectively encompasses three halls used for temporary exhibitions, a library and a cafe. Exhibitions focus on international architectural developments, and change on a regular basis. The extensive library is open to the public 10am to 5.30pm Monday, Wednesday and Friday and until 7pm on Saturday and Sunday. The centre also organises regular walking tours through Vienna on Sunday (in German), covering various architectural themes. You need to book ahead; see the website for dates and prices.
