This introductory chapter will evaluate how we have reached the current point in the history of world urbanity, its relationship with nature, and why a fusion between the two is now necessary. In order to define BioCities as cities which follow the principles of natural ecosystems to promote life, we will refer to the extensive knowledge of the history of urban science, the need for cities to be reinvented based on ecological principles, and new methods of analysing and measuring reality through digital systems. This vision of the main functions and traits of BioCities will also serve as a thread and reference for the subsequent chapters which will highlight and elaborate on the different properties of the BioCity vision. The final chapter will draw from this vision the constituting principles of the BioCity and will outline possible pathways of transition towards BioCities.