Urban design – the shaping of public space – is the most permanent of built environment practices, much more than buildings and plans that are demolished or abandoned with economic and political cycles. Yet many aspects of urban form change on a temporary basis, and a great deal of the potential for urban intensity involves the exploitation of both the temporal and spatial interstices of the city. Such interim and interstitial practices are generally also incremental and tactical in the sense that they squeeze between and within larger-scale strategies. There is now a pervasive global trend towards the temporary and the tactical that has become one of the key urban design strategies of the twenty-first century. Such projects range from guerrilla gardens, crosswalks, parklets and bike lanes to more formalised temporary beaches and swimming pools, parklets, instant plazas, pop-up buildings, food trucks, outdoor theatres and container towns. This emerging field of practices is difficult to define, it is described here as an intersection of the temporary and the tactical. Such practices enable different forms of social and artistic expression that are otherwise repressed in the overdetermined city. By enabling greater use of under-utilised spaces, they make urban space more productive and get more value out of the same infrastructure. They add intensity to the city through a greater diversity and adaptation of use and meaning plus a greater density of interaction. Temporary/tactical urbanism enables an increased level of creativity and innovation in urban design because it turns the city into a testing ground where new forms of thinking can be implemented without the danger of permanent failure. The temporary framework is a key to bypassing the formal planning processes that are necessary to secure approval for permanent change and build tolerance for innovative design.