Distributed networks of thousands of collaborating microsensors promise a maintenance-free, fault-tolerant platform for gathering rich multidimensional observations of the environment. Because a microsensor node must operate for years on a tiny battery, researchers must apply innovative system-level techniques to eliminate energy inefficiencies that would have been overlooked in the past. In this article we advocate two particular enablers for energy conservation: the ability to trade off performance for energy savings within the node, and collaborative processing among nodes to reduce the overall energy dissipated in the network. New levels of energy efficiency - attained through global system-level perspectives on node and network energy consumption - will enable a future where networks of hundreds, thousands, and eventually many millions of collaborating nodes are as commonplace as today's cellular phone.