Digital Twins (DT) has been added as an entry to the International Encyclopedia of geography: People, Environment and Policy. DT is defined as a cutting-edge approach to integrating research, development, innovation, and problem
solving using digital technologies. Using the COVID campus reopening as an example, DT emphasizes analyzing and addressing real-world challenges (safely reopen campus during the pandemic) by creating simulations, using data from sensors, satellites, and monitoring stations. These simulations enable predictions and testing potential solutions (number of people to bring back on campus and what intervention policy to deploy), thus, to inform decision-makers in safely reopen campus, and in many applications such as urban planning, environmental protection, and disaster management, global conflict, economy. This process facilities efficient and risk-reduced responses, which can be implemented either automatically or manually. (The research was funded by NSF COVID rapid project and NASA Goddard CISTO membership.)