In this paper, I propose to explore a new way of designing and experimenting with the city. How are we to conceptualize the changes in contemporary cities on the basis of their ambiances? What about the sensory and emotional production of urban territories? What happens when our aim is no longer just to design space but also to install an atmosphere? Such questions are situated at the intersection of issues of a social, aesthetic, urban, ecological and political nature. I should like to advance a hypothesis: we are currently witnessing what I call the ‘setting of ambiance’ in urban spaces. To give an account of this phenomenon, I do not intend to offer a formal definition of what an ambiance may be, but rather to show from what it proceeds, on what it is based, what it produces and transforms in urban life. Several ambiance operating modes in the province of urban design will be the focus: establishing the sensory as a field of action, composing with affective tonalities, giving consistency to urban situations, maintaining spaces over time and playing with imperceptible transformations.