Journal of Networking Technology Mahnaz Koupaee, Mohammad Reza 9 2 2018 http://www.dline.info/jnt/fulltext/v9n2/jntv9n2_1.pdf Recent advancements in sensor technology, wireless networks and consequently wireless sensor networks and the increase of their applications in different fields have led to their great importance. One of the most important challenges is the management of huge amount of data produced by sensors in network in order to reduce data traffic in network and minimize the energy consumption. This paper gives a survey on different data fusion techniques dividing them into two main groups: structured and structure-free. The main properties of structured approaches are discussed and the motivations to introduce structure-free approaches are emphasized via describing the shortcomings of structured approaches.