Urban lifestyles characterized by high consumption levels, exuberant use of natural resources, excessive production of waste, a widening gap between rich and poor, and a rapid growth of the human population pose a major problem for the future of the species. Therefore urban development must meet the needs of present generations without compromising the needs of future generations. This work introduces "sustainability assessment", a concept that aims to help in steering societies in a more sustainable direction, and applies this concept to cities.