The deployment of sensor networks in security- and safety-critical environments requires secure communication primitives. In this paper, we design, implement, and evaluate a new secure routing protocol for sensor networks. Our protocol requires no special hardware and provides message delivery even in an environment with active adversaries. We adopt a clean-slate approach and design a new sensor network routing protocol with security and efficiency as central design parameters. Our protocol is efficient yet highly resilient to active attacks. We demonstrate the performance of our algorithms with simulation results as well as an implementation on Telos sensor nodes.