We have developed an extremely low-power and low-cost wireless sensor network aimed at applications in asset tracking and ubiquitous activity monitoring. In addition to identifying an object, these nodes (termed active radio-frequency identification [RFID] tags) employ sensors to monitor its state, enabling new applications in fields like security, home automation, and supply-chain management. Although they contain a battery, these tags are not limited by it: by minimizing power consumption and quasi-passively waking on diverse stimuli (changes in light, RF carrier presence, shock or acceleration, and sound), they can last for years. Furthermore, their low cost and small size make them a good candidate for large-scale experiments at the intersection of RFID and wireless sensor networks.