Today 50% of the global population lives in cities, and this will grow to 70% by 2030. The rate at which cities and megalopolises are growing has outpaced the ability of city designers to anticipate their impact on the local and regional environment and populations. Consequently today's cities are becoming increasingly costly and unhealthy. Mature areas of computational science, ranging from microbiology to climate, have much to contribute to the design of new cities and the evolution and growth of existing cities.