“Smart City” has become a buzzword. Much is being written about smart cities as we speak, most of it promotional and uncritical. The goal of this article is not to criticize smart cities, nor is it to promote them. Rather, we would like to make a contribution to the conceptualization of smart cities and, by doing so, help the concept to become intellectually more solid. We think that this will also contribute to developing a more realistic and ultimately more practical view of what smart cities can achieve ... and what they cannot. In this article, we will proceed along the following five steps: in a first step, we will conceptualize cities as complex and dynamic socio-technical systems, a conceptualization without which “smart cities” – i. e., the penetration of cities by the information and communication technologies (a phenomenon also called digitalization) – cannot really be understood. In a second step, we will then define such digitalization much more precisely. Such a definition will be necessary in order to understand, in a third step, what digitalization exactly does to cities. In a fourth section, we will then discuss what such digitalization means for cities. In a fifth section, we will further elaborate on this and discuss the different perspectives on smart cities made possible by the digitalization of urban systems.