Complex Event Processing (CEP) is a relatively new, but got wider acceptability due to its systematic and multi level architecture driven concept approach. CEP is an emerging technology for processing and identifying patterns of interest from multiple streams of events. CEP is used in development of applications which have to deal with voluminous streams of incoming data with the task of finding meaningful events or patterns of events, and respond to the events of interest in real time. In this paper a CEP based application for object detection and tracking in a Wireless Sensor Network (WSN) environment is proposed. Also the detection of an intruder using semantic query processing is proposed. © 2010 IEEE.