In the coming decades, we will live in a world surrounded by tens of billions of devices that will interoperate and collaborate in an effort to deliver personalized and autonomic services. This paradigm of objects and things ubiquitously surrounding us is called the Internet of Things (IoT). Cities may be the first to benefit from the IoT, but reliance on these machines to make decisions has profound implications for trust. Trusting smart cities refers to the confidence and belief of smart city installations to be capable of operating securely, reliably, and accountably. In order to understand how trust applies to smart cities, we introduce formal definitions of trust and risk, and present three risk factors that capture the range of issues that must be considered when deploying smart city technologies. Building on these risk factors, a threat analysis matrix for capturing how well smart cities are addressing these risks is proposed.