Chapter 3 puts forward an original conceptual analysis of security. It argues that security is a relational concept. To understand any discussion of security, one must hold certain pieces of information, including (a) security for whom (an agent); (b) security of what (a value or interest); (c) security against what (a threat or risk); and (d) security by whom (a provider of protection). It is argued that the concept of security is silent about the level at which it is applied (international, national, group, individual); silent about the interest or value to be secured; silent about the type of threats and risks which are relevant; and silent about who should provide the protection and how. It is therefore context-sensitive. It follows that contextual discussions of security rely upon a combination of political theory and policy decisions to determine which interests or values should be preserved or promoted, against which risks and threats, how, and by whom. This makes security an adaptable concept and helps to explain why it has come to be used in so many different senses and such diverse fields.