Biocities are smart, attractive and sound cities interconnected with the forest and forestry approaches, and where human communities adopt nature-based solutions – and develop a circular bioeconomy for the mutual wellbeing of environment and people. The fields of application of the Biocity concept include the sustainable management of wood sources, the use and re-use of bio-materials from forests (especially timber) and other ecosystems, the development of biobased nano-materials, a new urban metabolism using renewable systems in the field of energy, building materials, the water cycle, the production of food in cities, the balance between urban biodiversity and resident perception, and the improvement of air and soil quality. Europe has defined the New Green Deal (NGD), significantly confirmed after Covid-19 pandemic, to achieve the Zero Emissions Europe by 2050. Biocities can bring about a change in the socio-economic paradigm based on the relationship between the circular bioeconomy, the natural systems spread over the European territory, and the urban metabolism to achieve healthy, inclusive, and resilient cities. According to the United Nations, there will be 10 billion people in the world by 2050, with an urban population of 68%. Thus, in the next 30 years, the equivalent of a city of 6.6 million people will be created in the world every month. Therefore, the best way to develop a sustainable earth is to rethink the urban model, rewilding degraded landscapes and halting fossil fuel use. Being the continent with one of the longest urban tradition, Europe should take the lead in the development, and become an example and a template for global implementation. In order for this to happen, it is paramount that any future initiative will focus on multiple scales from micro to regional, adopt socio-ecological, socio-economic and new governance approaches also in light of the recently communicated EU biodiversity strategy. The innovative value of the Green Book of Biocities is a novel synthesis of previously disparate disciplines, all contributing to life in the urban environment (i.e. forestry, ecology, agriculture, climate science, energy engineering, environmental accounting, material science, development science, infrastructure engineering, architecture and urbanism, big-data science, geotechnics, policy, governance, social science, public health and economics) to generate a distinct, evidence-based holistic framework.