"Digital twins" are virtual representations of physical objects, processes, or systems—like factories, traffic patterns, and even people. A digital twin is used to predict how changes may affect its physical counterpart. For example, a city could use a digital twin to monitor its energy use to plan for extreme weather. Digital twins have benefits, like making manufacturing more efficient, but they can raise technical, security, and ethical questions. For example, a doctor could use a patient's digital twin to detect health problems, but there may be data privacy issues. We reviewed some of the opportunities and challenges of this technology.