Tactical urbanism initiatives have been interpreted as an alternative and a challenge to formal spatial planning tools to the need for a more responsive planning system. This paper analyses tactical urbanism initiatives in the United States considering three main aspects: the process, its interaction with planning institutions and the respective urban design outcomes. For this, the relation between tactical urbanism and complexity theory (in which self-organisation and evolution play an important role) is addressed. Findings suggest some contributions that tactical urbanism can make to urban design and spatial planning, in evolutionary terms and possible role for tactical urbanism in alternative to traditional division between plan making and plan implementation.