Some planetary boundaries have been crossed, making it urgent to act on climate change and biodiversity, among others. We are at a crossroads where imagining a desirable and possible future is necessary. Acting on sustainable cities and communities falls under the 11th UN SDG and responds to the growing challenge of urbanization worldwide. In this Special Issue, we invite multidisciplinary contributions on current trends and perspectives in ecosystems, innovations, models and (possibly replicable) good practices in smart cities and territories that improve the environment, the economy, and social wellbeing. Advances in simulation methods and data analysis, forecasting techniques, scenario planning, future design, and complex system models are also particularly welcome as long as they can participate to improve critical thinking and decision making on cities’ and territories’ resilience and sustainable development.