The author argues that conflicts caused by the distribution of externalities -externalities which stem from changes in land use- constitute major challenges but, at the same time, unique opportunities for urban planning. Paying attention to these conflicts would enhance a form of planning suitable for the coping of two important current challenges in big cities: that of achieving environmental sustainability, and that of establishing politically more sophisticated and successful management styles than traditional ones. Conflicts caused by externalities are spreading out with economic globalization, urban growth and the rise of environmental consciousness. Ther are not merely environmental conflicts, but also social conflicts for the control of local territories; and, from the viewpoint of local communities, they represent the defense by people of their quality of life, threatened by real estate, industrial or public works projects. These conflicts have an undeniable political-economy, or distributive, component to them. Finally, the author claims that the handling of these conflicts offers a way for the political enhancement of urban planning and, at the same time, a contribution for the reconciling of the latter with the ecological concern that modern urban planning had in its inception, but abandoned along the years.