IntroductionThe quality of life in cities and regions depends strongly on the accessibility of their population to services such as education, health care, public safety, and justice. For this reason the decisions regarding the location and size of the facilities associated with those services need to be carefully planned.Very often decisions of that kind are made in the context of spatial development plans through a comprehensive approach. When this is the case, rather than defining whether a particular facility should be located in a particular urban center in some region, planners will typically aim at defining the region's urban hierarchy öthat is, the level of hierarchy to assign to the urban centers of the region, each level being characterized by a class of facilities. For instance, in a three-level hierarchy, facilities such as elementary schools or basic health care units could be associated with firstlevel centers, whereas secondary schools and local hospitals could be associated with second-level centers, and universities and central hospitals with third-level centers. Note that centers at the higher levels of hierarchy are typically assumed to have all lower level facilities as well (or, in the case of facilities such as hospitals, to be able to provide all services available at lower level facilities).The decision to elevate an urban center to a new level of hierarchy implies a large investment of public funds, opening of new job opportunities, relocation of associated`spin-off' industries, etc ö all of these factors likely leading to significant increases of population at the`elevated' centers, with corresponding future increases of demand for services. Of course, these increases in population are likely to be due