The political objective calling for environmentally sustainable development has provided the canvas for lively dialogue amongst scientists and analysts. Be that as it may, what have been found wanting are operational conditions for the achievement of environmental sustainability within specific anthropogenic systems, with cities being cited as a prime example of such systems. Indeed, from a historical perspective, cities have always been based on the concentration of humans and man-made elements, resulting in respective confinement of natural assets. As cities possess particular properties for their environmental basis, the general conditions governing sustainability require suitable modifications when applied to cities. In this context, the present paper examines the conditions for environmentally sustainable development in urban systems. The specific natural and socioeconomic characteristics of urban systems are systematically taken into account for defining environmentally sustainable development in cities. The paper proposes an operational framework for evaluating urban systems for their environmental performance. Urban systems, presenting specific characteristics necessary for creating welfare with high density, ask for particular conditions for their environmentally sustainable development. These conditions differ substantially from those defining sustainability in broader geographical systems. Copyright © 2006 John Wiley & Sons, Ltd and ERP Environment.