This article examines interstices in the urban fabric using the example of two urban leftover spaces in Paris. The article first analyzes the institutional mode of treating these spaces, which explains the ‘framing’ of the interstice as a temporary functionless space. It shows how interstices are not only institutionally created and controlled, as opposed to free, but also find a functional place as a temporary margin of maneuver in a process of decay, recycling and renewal enforced by landlords, the police and maintenance teams. Second, the article examines the improvised modes of action developed by diverse people in order to use the interstice. The article looks at what happens in the gaps of urban planning, when activities find a place in the interstice not in order to transform it, and bring it back into the realm of urban places, but to take advantage of its ‘in-between’ position in the city. In practice, such activities are led by individuals who have to be ‘just passing’, because the frame (Goffman, 1974) built by landlords and their agents prevents them from taking place. Under some conditions, ‘just passing’ can give way to another type of involvement described as ‘out of frame’, which in this case, allows a group of homeless people to settle in the interstice for a more durable period of time despite heavy surveillance.