Em* (pronounced EmStar) is a software environment for developing and deploying Wireless Sensor Network (WSN) applications on Linux-class hardware platforms (called ''Microservers''). Em* consists of libraries that implement message-passing IPC primitives, tools that support simulation, emulation, and visualization of live systems, both real and simulated, and services that support for networking, sensing, and time synchronization. While Em*''s design has favored ease of use and modularity over efficiency, the resulting increase in overhead has not been an impediment to any of our current projects.