Wireless sensor networks (WSNs)  are massively distributed systems for sensing and processing of spatially dense data. They are composed of a large number of nodes deployed in harsh environments to execute challenging tasks including security/surveillance, environmental monitoring, health monitoring, industrial automation and disaster management, etc. Although the nodes only have limited resources, complicated tasks such as distributed detection and estimation  can be accomplished via nodes’ cooperation. The main argument is that a distributed sensor network can leverage its performance by aggregating information gathered by individual nodes, which is known as information fusion. The primary goal of sensor fusion is to process and progressively refine information from multiple nodes to eventually obtain situation awareness.