Nano-RK is a fully preemptive reservation-based real-time operating system (RTOS) from Carnegie Mellon University with multi-hop networking support for use in wireless sensor networks. Nano-RK currently runs on the FireFly Sensor Networking Platform as well as the MicaZ motes. It includes a light-weight embedded resource kernel (RK) with rich functionality and timing support using less than 2KB of RAM and 18KB of ROM. Nano-RK supports fixed-priority preemptive multitasking for ensuring that task deadlines are met, along with support for CPU, network, as well as, sensor and actuator reservations. Tasks can specify their resource demands and the operating system provides timely, guaranteed and controlled access to CPU cycles and network packets. Together these resources form virtual energy reservations that allows the OS to enforce system and task level energy budgets.