Simulation can be an important step in the development of software for wireless sensor networks and has been the subject of intense research in the past decade. While most previous efforts in simulating wireless sensor networks have focused on protocol level issues utilizing models of the software implementation, a significant challenge remains in precisely measuring time dependent properties such as radio channel utilization. One promising approach, first demonstrated by ATEMU, is to simulate the behavior of sensor network programs at the machine code level in a cycle accurate way, but poor performance has so far limited its scalability. In this paper we present Avrora, a cycle accurate instruction level sensor network simulator which scales to networks of up to 10,000 nodes and performs as much as 20 times faster than previous simulators with equivalent accuracy, handling as many as 25 nodes in real time. We show how an event queue can enable efficient instruction level simulation of microcontroller programs and allow the hidden parallelism in fine grained sensor network simulations to be extracted, once two core synchronization problems are identified and solved. Avrora s ability to measure detailed time critical phenomena can shed new light on design issues for large scale sensor networks.