This research project outlines some methods and tools developed in order to integrate knowledge into various processing chains implemented for defence and security applications. Project contributions focused on several aspects. First, assessing the quality of information provided by humans, for which two main approaches were developed relying either on the semantics of ontologies to detect inconsistency and contradictions or on the properties of sources in the reporting chain to assign a degree of trust to information items. Second, the use of semantics as a backbone for information retrieval, reuse of experiences, and data fusion in heterogeneous and dynamic environments has been investigated. Those approaches contribute to the overcoming problem of dynamically integrating resources, user feed-back, sensor data and observations, which is a critical aspect of situation awareness today. Third, the exploitation of open sources and social data in the context of defence and security applications has been addressed. Those methods have been developed in collaboration with several fellow researchers and include contributions from master thesis and post docs. The work was carried out in the framework of several research projects, with funding from the European Commission, ANR and CNRS. The projects involved academic and industrial partners, such as: George Mason University (USA), DRDC (Canada), FKIE (Germany), NATO CMRE (Italy), University of Paris Nanterre, University of Toulouse II (France), Thales TNO (Netherlands) and Airbus D&S (France).