There is a lack of clear and systematic definition of physical security in Korea, so its basic concept has not been well-established. In an attempt to fill this gap, the purpose of this study is to investigate the ‘Definition of Physical Security’ and provide the theoretical basis for research conducted in the area. The ‘Risk Assessment’ theory is applied and based on the three determinants of the magnitude of risks (i.e. asset, threat, and vulnerability), the scope of physical security control and related problems are analyzed.In defining physical security, the scope of ‘Asset’ includes people, information, and property. In regard to ‘Threat’, intentionality is the main consideration. Additionally, structural, electronic, and human factors are taken into account for ‘Vulnerability’. After examining the scope of each determinant, the following definition of physical security is derived.“Physical security is defined as the efforts to control physical vulnerabilities in order to secure people, information, and property from intentional threats such as crimes. The control measures consist of structural components such as buildings or security-related facilities, electronic components including security systems, and human components like security officers.” Key words: Security