When wandering through European cities, one can detect an increasing prevalence of autonomous appropriations of public space by citizens. The smallest of spaces such as tree pits, niches, and pavements, as well as larger sections of parks, are being taken over and shaped creatively. This do-it-yourself mentality of city residents results in alternative, authentic, and vibrant open spaces, which stand in contrast to our planned city. The projects are informal, self-built, provisional, and create new social spaces. They intervene temporarily or long-term and make use of the given circumstances as they are found. What is happening here can be seen as an open source or laymen’s design of informal open spaces, or as an autonomous form of city planning. This phenomenon is presented using examples of civic initiatives for the appropriation of public spaces in Berlin, Hamburg, and Zurich. Alongside 14 inspiring principles, five chapters provide valuable tips and advice on matters such as the generation of ideas, location scouting, legal issues, procurement of materials, and financing. They are intended to motivate people to venture their own interventions into the cityscape, thereby contributing to shaping their urban living environment.