With the increasing demand for more complex and larger models in different fields, such as the design of virtual worlds, video games, and computer animated movies, the need to generate them automatically has become more necessary than ever. Manual tools are no longer sufficient to match this rising need, and the impact that automatic tools may have within these fields is essential and may lead to an adoption of virtual worlds in a growing number of applications. Indeed, it is possible to eliminate most of the effort associated with the creation of such environments, by providing tools that may generate “massive” 3D content automatically. In consequence these tools may lead to an exponential growth of virtual environments and represent an important turn into the design of realistic virtual cities, which may have a huge impact on virtual world users. This chapter discusses the very complex issue of where and when procedural modelling may be used and presents some solutions and methods that have been successfully used in the aforementioned fields of application.