This paper presents a systematic approach to analysing the visual impact of tall building evolution on cities and their surrounding landscape, using Rotterdam and The Hague as case studies. Critical tall building clusters that visually determine the skyline of both cities are identified and allow comparison of actual tall building development and the urban policies in place. The research demonstrates that a considerable distance exists between policy and reality. Both Rotterdam and The Hague struggle to deliver a consistent and integrated policy for tall-rise urban areas, while tall building developments seem to be ruled by an internal logic not fully recognized in policy-making. Using the visibility of the skyline to identify tall building clusters suggests that both cities could allow developments in a much wider area than originally envisioned in their guidance on tall buildings. Although each new tall building design faces public and political scrutiny, the fact is that the visibility pattern in both cities is already established. Each new development has a decreasing impact as long as it is confined to the established tall building cluster. As shown in the paper, GIS-based visibility analysis is a powerful tool for tall building planning and design, not only increasing understanding of actual developments and their effects in a precise and quantifiable manner, but also helping to evaluate and develop tall building policies.