Two cities and two themes: voids and density. In one city, voids have been expressly created, in the other the voids are the result of decline. In one, voids are defined by the surrounding density, in the other the empty spaces neither define nor are defined. Milan and Detroit have very little in common. Indeed they seem opposites, like positive and negative images of the same picture. The voids in Detroit amount to some 60% of Milan’s surface area. On the one hand there is Milan with its Urban Development Plan, on the other, a social phenomenon. A new city is being planned for Milan with new building typologies that don’t require further land consumption; in Detroit, attempts - including urban agriculture projects - are being made to stem an exodus that has created a void as large as the surface area of San Francisco and twice that of Manhattan. In Milan, they are gearing up for a world expo while in Detroit the focus is on redefining the urban experience in a hollowed out city. Yet the two metropolises have features in common. Both have a history of industrial production and innovation and as economic capitals, which although a thing of the past for Detroit, nonetheless remains very much part of the collective psyche and a source of a singular civic pride. Both are strategically located within their respective wider regions. Both were nurtured on the common ethos of being the production powerhouses of their respective countries. And even today, despite their divergent situations, this keeps alive the sense of being a great city, almost with a mission to portray a way of life. Milan and Detroit are at extremes. Yet between them spans a range of important questions that impact all contemporary metropolises: what are the mechanisms that create virtuous cycles linking civil mobilisation, urban design and city management; how to combine appropriation and spontaneous regeneration of new urban landscapes with guided urban planning and architecture; how to include the new requirements of its citizens; how to calibrate private investments and public administration goals, and what sort of public/private partnership is best? Then there are the issues of the voids. How should these be utilized (or created) to promote an overall urban design and offer new public space opportunities for the future? What sort of density should there be around and inside the voids, and what is the best way to use the huge brown sites left in a post-industrial city? How can these voids be the focus for a new urban structure and successfully implement the more complex, intricate models of today’s cities (lattice, radial, concentric?) in contrast to the single-matrix formats of the past? How can these voids be the pivotal points for a diverse range of ad hoc programmes at the different urban scales to develop a new way of city living? How can areas of urban intensity (not to be confused with building density) be created inside the voids rather than around them? These are issues that concern not only Milan and Detroit but also many post-industrial Western cities. While European cities of the “Europe of the Regions” enjoy greater density and a robust infrastructure network, they also have to contend with more environmental, spatial and cultural constraints. American cities in contrast have greater land availability and an urban fabric still fairly receptive to transformation. On the downside, their much-proclaimed regionalism has rarely become autonomy of governance and their infrastructure is sorely underdeveloped – remaining one of the most difficult challenges for the Obama administration. Between Milan’s new Urban Plan and the non-plan that is the Detroit Works Project lies a series of attempts to provide answers to the essential issues of the day. The guiding vision put forward by Metrogramma with Milan’s new Urban Development Plan is posited on a new urban humanism and a new meaning of urban quality for the contemporary metropolis. What strikes one about Detroit, on the other hand, is the determination of its citizens, organizations, architects, operators and administration to come up with intervention strategies to avert the pernicious cult of an “aesthetic of decay”. There is, quite rightly, a general move against what some see as Detroit’s inevitable destiny as the “American Pompei”. There seems to be a growing awareness that urban design is not enough to solve the huge and serious problems of an imploded city. Importantly, however, Milan and Detroit are linked by a common thread: the cities’ own commitment to understand, imagine and make happen a new identity as environmental systems rooted in place, context and a specific culture. In Milan this is taking place perhaps in a more structured manner through the action of the local administration guided by a carefully drafted urban design plan. In Detroit, action is perhaps more spontaneous, based on the activism of neighbourhoods or “urban explorers”, and with a series of inevitably more piecemeal projects. Yet there is a common positive signal to be seen in the parallel stories of these two cities: the belief in a new urban culture shown by citizens themselves and their willingness to take an increasingly decisive role in their city’s destiny.