Urban areas across Europe are constantly expanding under the pressure of demographics and of ever-increasing mobility. Meanwhile, the institutional boundaries of the various communities that make up these urban areas remain remarkably stable. The resulting mismatch between a functional urban area and its territorial governments is not new and is liable to increase. It makes the coherent planning and development of the urban area extremely difficult, if urban planning remains within the remit of the various local communities. The dilemma of resolving this mismatch has led to many contributions to the scientific literature since the start of the twentieth century. All this work has not delivered a consensus, nor easily applicable solutions. Some of the theoretical contributions have sought to address the values that should be devolved to the local level and to its government. Others have probed whether it makes sense to try to make the various types of boundary coincide. Despite the pitfalls, institutional reform has been carried out at territorial level in many European countries. The federal policy on conurbations, launched in Switzerland at the beginning of the 21st century, is a case in point. The roll-out of the federal policy on conurbations across a majority of urban areas in Switzerland has led to the creation of strategic urbanisation plans at regional scales, extending well beyond the boundaries of a single town. Most of these projects remain to be institutionalised, i.e. transcribed into legally binding documents for urban planning. However, the federal policy on conurbations did not change the prerogatives of the various institutional levels. The actual implementation of a conurbation-level urbanisation plan remains in the hands of the various local communities in the area, and is therefore carried out using the same procedures and by the same actors as before. Under what circumstances and with what effects is this crossing of institutional lines taking place? This question is at the heart of this research project. Within a given community, the aim is to understand how the emergence of conurbation-level planning leads to a change of practice at local level. At the conurbation level, the objective is to identify how new cooperative mechanisms are being put in place to enable local actors to work together on a common project – with the inevitable successes and failures. The analysis is operationalised by investigating urbanisation plans in several conurbations in Switzerland. The emphasis is on reconstituting the complex process whereby the planning processes were set up and on understanding the types of governance that were set up to oversee them. A comparison between different settings, as well as with urbanisation plans that do not cross community boundaries, yields information with general applicability. Furthermore, this research calls into question the idea of equating a conurbation’s functional area with its institutional boundaries through urban projects. It demonstrates the limits of such an approach. More generally, it shows the considerable influence of local-level values on the behaviours and procedures carried out at the level of the conurbation. Beyond Switzerland, this research seeks to add to the global knowledge base regarding the capacity of local communities to cooperate and interact within strongly fragmented conurbations – with a view to forming collective responses to territorial challenges.