This paper is the initial result of a cross-disciplinary attempt to encircle an answer to the question of optimal densities of sustainable settlements. Urban density is an important component in the framework of sustainable development and influences not only the character and design of cities but also the architectural and technical strategies of buildings. The paper is based on literature studies in the fields of urban mobility, urban ecology, urban design, architecture and civil engineering. It concludes that neither detached houses nor extremely dense cities are optimal answers to the sustainable challenge.