Network Based Defence (NBF) has been introduced in Sweden to provide military personell with rapid and reliable Situational Awareness. Networks are flexible and robust structures that can function even if some nodes are lost. This property makes the network less sensitive to traditional forms of electronic warfare, which are directed at individual sensors and communication links. The project "Electronic warfare against NBF" was initiated to study new forms of electronic warfare that may be used against Network Based Defence. The analysis shows that it is advantageous to attack the decision process by flooding the system and applying deception. These methods are cheap to use and can be initiated at short notice. The effect of electronic warfare is analysed by developing a theory which is applied to several different cases. The theory describes the data obtained from sensors and the knowledge of operators by representing them as choices and assigning probabilities in terms of information. This view of Situational Awareness can be used to classify various forms of electronic warfare and assess their effectiveness. Deception is shown to be a particularly effective method and can be applied even at the tactical level.