The 20th century has rapidly changed life around the world and initiated an unprecedented urbanization that continues unrestricted today in the 21st century. Throughout the world, urban growth was fueled by migrations, both within and between nations or regions and in particular between rural sides and cities. While this development started in the Global North, the second half of 20th century was increasingly influenced by international trade and technological developments that further stimulated urbanization, in particular in South America, Asia and Africa - the Global South. Around 1950 more than half of the 17 large cities with a population of more than one million were in Europe. By 2000, the number had grown to 387, with more than half in Asia and for 2100 the majority is expected in Africa. Across the globe cities grew rapidly and became megacities. As a result, cities worldwide increased in size, population, and density and created new forms of urban living: densification and suburbanization, which is also characterized by separating the functions of living, working, shopping and leisure. Automotive dominated mobility, urban mass transportation, infrastructures and utilities and large housing estates evolved as new typologies in architecture and the built environment. As modern heritage they represent the legacy of the 20th century. With regard to the scarce planetary resources and to safeguarding the quality of life, the planning and design approaches to this modern heritage will become a key factor and guarantee for economic, ecological and social stability in the cities and remains a challenge for politicians, designers and the people themselves. Global strategies in combination with regional and local approaches will be needed to value, preserve, densify and re-use this built heritage. in order to enable urban surfaces and spaces to being harnessed in new ways, contributing to new forms of living and working, daily food needs, energy supply or to improving the microclimate of cities.