Distributed networks of thousands of collaborating microsensors promise a maintenance-free, fault-tolerant platform for gathering rich, multi-dimensional observations of the environment. As a microsensor node must operate for years on a tiny energy source, innovative energy management techniques are needed. Widespread device deployment makes battery replacement infeasible, requiring energy to be scavenged from the environment-- e.g., conversion of ambient vibrations to electric energy. Computation and communication must be optimized for very low duty cycles, making issues such as standby leakage and start-up overhead critical. All levels of the communication hierarchy, from the physical and link layer to routing protocols, must be tuned for energy efficiency. A total-system approach is required for reliable, self-powered microsensor networks that deliver maximal system lifetime in the most challenging environments.