What is a city? What is an urban system? Do we understand these most complex man-made artefacts in their entirety? Why do people move into cities? When do they prefer to stay in rural areas? Do cities need skyscrapers? Are there cities without density or are there dense settlements without being a city? Some cities are liveable for the majority, others just for a few. As we enter the first urban century, we start to realise that today‘s cities are not sustainable, no matter from which side we look at them. Prerequisites for their transformation towards liveability, sustainability and resilience are better knowledge and ability to change. Understanding the city and knowledge about the city should be the base for change. As we begin to realise that cities are not neutral objects, but that people define the city, the mobile citizen gains a central role in the definition of the future city.