This book presents atheory as well as methods to understand and to purposively influence complexsystems. It suggests a theory of complex systems as nested systems, i. e. systems that enclose other systems and that are simultaneously enclosed by evenother systems. According to the theory presented, each enclosing system emergesthrough time from the generative activities of the systems they enclose. Systems are nestedand often emerge unplanned, and every system of high dynamics is enclosed by asystem of slower dynamics. An understanding of systems with faster dynamics,which are always guided by systems of slower dynamics, opens up not only newways to understanding systems, but also to effectively influence them. The aim and subjectof this book is to lay out these thoughts and explain their relevance to thepurposive development of complex systems, which are exemplified in case studies from an urban system. Theinterested reader, who is not required to be familiar with system-theoreticalconcepts or with theories of emergence, will be guided through the developmentof a theory of emergent nested systems. The reader will also learn about newways to influence the course of events - even though the course of events is,in principle, unpredictable, due to the ever-new emergence of real novelty.