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The human habitation of the surface of the planet has led, especially since the mid-twentieth century, to an enormous increase in the built up area. This phenomenon concerns both the oldest industrialised countries, such as European Union and the United States, and the so-called emerging countries in Asia, Africa and Latin America.The urbanised built area has increased in different ways, but has led everywhere to the destruction of large portions of virgin soil, the loss of biodiversity in the number and type of species in fauna and flora, and often the total or partial impairment of ecosystem functions of enormous environmental value (such as the evapotransportive mechanisms of soil and vegetation). Although the curve of global population growth is slowing, the growth of urban areas continues to expand, even in countries which have long been industrialised, where the spread of building has given rise to cities dispersed over a territory so that there is no longer a recognisable clear division between the city and the countryside. In fact, contemporary cities are the main consumers of all environmental resources, from water to food, including energy and environmental stressors, and are responsible for 80% of CO2 emissions into the atmosphere. The ecosystem surface required to sustain a large city on the planet today canbe about 200 times larger than its physical size. The urban civilisation of the 21st century, which at a superficial glance may appear as a symbol of the human capacity to radically adapt and transform the natural habitat toits own needs, is also a witness to the unsustainability of the human footprint on earth. Radically rethinking cities and human settlements entails an equally radical rethinking of our economic and development model, but it is a necessary and strategic task if we really want to face the challenge of sustainability with appropriate instruments.

This paper’s focus is to reflect on the global growth of urban areas, according to number and size of inhabitants and also according to the environmental, social, cultural and economic impact of this phenomenon. If city growth is an issue which addresses mainly developing countries such as China, India or the states in South America, the features acquired by urbanisation in the OCSE countries, currently undergoing economic and demographic contraction, lead towards a territorial dispersion of the built up areas with serious consequences especially in terms of soil use, pollution and loss of site identity. Thus, the solutions proposed by the various national and supranational programmes intended to guide global urbanisation to a greater sustainability must be differentiated between countries with fully developed economies from those whose economy is currently on the rise, and must also actively involve the local communities.