Informational cities are prototypical cities of the knowledge society. If they are informational world cities, they are new centers of power. According to Manuel Castells (1989), in those cities space of flows (flows of money, power, and information) tend to override space of places. Information and communication technology infrastructures, cognitive infrastructures (as groundwork of knowledge cities and creative cities), and city-level knowledge management are of great importance. Digital libraries provide access to the global explicit knowledge. The informational city consists of creative clusters and spaces for personal contacts to stimulate sharing of implicit information. In such cities, we can observe job polarization in favor of well-trained employees. The corporate structure of informational cities is made up of financial services, knowledge-intensive high-tech industrial enterprises, companies of the information economy, and further creative and knowledge-intensive service enterprises. Weak location factors are facilities for culture, recreational activities, and consumption. Political willingness to create an informational city and e-governance activities are crucial aspects for the development of such cities. This conceptual article frames indicators which are able to mark the degree of “informativeness” of a city. Finally, based upon findings of network economy, we try to explain why certain cities master the transition to informational cities and others (lagging to relative insignificance) do not. The article connects findings of information science and of urbanistics and urban planning.