A digital twin is a digital representation of an object of interest and may require different capabilities according to the specific domain of application, such as synchronization between a physical thing and its digital representation, and real-time support. A smart city digital twin can be defined as a digital twin for a smart city that can be used to develop strategies to achieve specific goals for a smart city, by conducting simulations and to increase visibility of human-infrastructure-strategy interactions. A smart city digital twin allows the simulation of plans before implementing them, exposing problems before they become a reality. In other word, it is possible to conduct simulations on a digital replica of the city (virtual cities) before actually implementing the strategy on the real city. In this way, it is also possible to find the best strategies to achieve a specific goal or strategies that have similar effects while minimizing budget and resource usage. Therefore, a smart city digital twin is a tool for improving urban operations, efficiencies and resilience of a city.