In "The Right to the City" Henri Lefebvre states that urban praxis requires "places of simultaneity and encounters" that make room for the fluid, shifting relationships of everyday life and social interaction (Lefebvfre 1996). Designers cannot create these relationships; they come from the people who actually inhabit the city. Designers can, however, "clear the way" and "give birth to the possible" by creating opportunities for praxis to occur. This paper discusses how contemporary design activism realizes Lefebvre's "right to the city" through techniques rooted in historical participatory design. It presents examples from the work of Aldo Van Eyck and Lucien Kroll and builds on these with the work of the contemporary activist designers Teddy Cruz and Urban Think Tank. These designers approach design as a facilitator of social interactions that can be shaped to meet the needs of diverse users and generate new types of social and economic relationships. Designed as systems rather than objects, their projects are open-ended and flexible while remaining functional and they make use of the informal systems already operating in their communities. These projects not only serve needs through spatial infrastructure but also create opportunities for urban praxis by operating as social infrastructure.