So-called sensor nodes combine sensors, processors, wireless communication capabilities, and autonomous power supply in a tiny device. Large-scale networks of these untethered devices can be deployed unobtrusively in the physical environment in order to monitor a wide variety of real-world phenomena with unprecedented quality and scale. A fundamental service in sensor networks is the determination of time and location of events in the real world. This task is complicated by various challenging characteristics of sensor networks, such as their large scale, high network dynamics, restricted resources, and restricted energy. We develop new approaches for determination of time and location under these constraints, and devise design principles based on our experience. We illustrate the practical feasibility of our approaches by a concrete application.