Abstract In many sensor applications, the data collected from individual nodes is aggregated at a base station or host computer. To reduce energy consumption, many systems also perform in-network aggregation of sensor data at intermediate nodes enroute to the base station. Most existing aggregation algorithms and systems do not include any provisions for security, and consequently these systems are vulnerable to a wide variety of attacks. In this talk, I will discuss a particularly pernicious attack in which a compromised node falsifies the sub-aggregate transmitted to its ancestors in the aggregation hierarchy, and present efficient algorithms for computing the SUM and MEDIAN aggregates that are resilient to this attack. Bio Sanjeev Setia is a Professor and Chair of the Computer Science Department at George Mason University. He received his PhD from the University of Maryland, College Park in 1993. His research interests are in ad hoc and sensor networks, network security and performance evaluation of computer systems. In recent years, he has worked extensively on security mechanisms and protocols for ad hoc and sensor networks. He was the founder of the ACM Workshop on Security in Ad hoc and Sensor Networks (SASN) and served as its co-organizer in 2003 and 2004. His research has been funded by NSF, NASA and DARPA.