This module examines facilities as systems, focusing on the building. It identifies the interchanges a facility has with the external environment, and where potential physical, chemical and biological threats may occur. Chemical and biological agents are discussed, and the mechanisms of their plausible introduction and spread within the facility considered. Dose-response relationships are discussed as a means of providing understanding of the risks involved. Methods and technologies for alleviating the impact of security threatening episodes are discussed. Methods of assessing the vulnerability of facilities are introduced.