In many cities, urban wastelands and vacant structures suddenly metamorphose in exuberant places. After city planners and the real estate market have failed in their initial attempts to develop them, these sites become the setting for clubs and bars, start-up firms and art galleries, migrant economies and informal markets, recreational activities and nightlife. Indeed, it is often precisely here that innovative cultural production and a vibrant public sphere are to be found. The Urban Catalyst research team explored these unplanned temporary uses in five European countries over the course of several years, and did far more than merely analyze their hidden logic. Building on the team's own participation in a number of different projects, Urban Catalyst examines various ways in which city planning can incorporate informal processes and describes important lessons architects and city planners can learn from temporary users. Included are seminal contributions by Azra Aksamija, Kees Christiaanse, Margaret Crawford, Jesko Fezer, Florian Rotzer, Saskia Sassen, and others, as well as key projects from European cities such as Amsterdam, Basel, Berlin, London, Rome and Zagreb.