This paper extends the authors’ previous work on observabillity of discrete-event systems by taking time into consideration. In a timed discrete-event system, events must occur within their respective lower and upper time bounds. A supervisor can disable, enable, or force some events to achieve a given control objective. We assume that the supervisor does not observe all events, which is often the case in practice. We generalize the concept of observability to timed discrete-event systems and show-that it characterizes the existence condition for a supervisor. We also generalize normality, a stronger version of observability, to timed discrete-event systems, which has nice properties that are absent in observability. We then derive conditions under which observability and normality are equivalent. We propose two methods to synthesize a supervisor, a direct approach and an indirect approach. An example is given to illustrate the results.