This chapter provides a philosophical analysis of the different meanings of 'security' and, by so doing, identifies some key features of the concept of security. It explains a conceptual analysis of security with the aim of integrating the concept with moral and political theorising. Security has also become an important concept in deliberations on the response to infectious disease emergencies. The discipline of International Relations, where national security is a key concept, has built upon this Enlightenment heritage by associating the term almost exclusively with the territorial and political integrity of the State. The word 'security', however, has its etymological roots in the Latin 'securitas' which primarily denotes a kind of psychological quietude. In this sense we can think of an individual's objective security as what God might know about the reliability of that individual's protection from infectious disease. Related to the notion of reliability is the tension between the objective and the subjective realisation of security.