In this paper, we investigate one of the major challenges in pervasive systems: energy efficiency, by exploring the design of an RFID system intended to support the simultaneous and real time monitoring of thousands of entities. These entities, which may be individuals or inventory items, each carry a low-power transmit-only tag and are monitored by a collection of networked base-stations reporting to a central database. We have built a customized transmit-only tag with a small form-factor, and have implemented a real-time monitoring application intended to verify the presence of each tag in order to detect potential disappearance of a tag (perhaps due to item theft). Throughout the construction of our pervasive system, we have carefully engineered it for extended tag lifetime and reliable monitoring capabilities in the presence of packet collisions, while keeping the tags small and inexpensive. The major challenge in this architecture (called Roll-Call™) is to supply the energy needed for long range continuous tracking for a year or more while keeping the tags (called PIPs) small and inexpensive. We have used this as a model problem for optimizing cost, size and lifetime across the entire pervasive, persistent system from firmware to protocol.