Security is a core function of the modern nation state. The government has a duty to protect its citizens; to ensure their security and prevent them from coming to harm. The term security has traditionally been understood as pertaining to national security. However, in recent years, the locus of national insecurity has shifted away from external state threats to other sources of domestic harm. States have shifted their security gaze inwards, to protect their citizens from sources of internal insecurity such as civil wars, terrorism and political violence. State responses to these types of threats have typically fallen within a criminal justice, rather than a war paradigm. Consequently, security measures and preventive measures have gained prominence in relation to criminal law and criminal justice and a new literature, focusing on ‘the preventive state’ and preventive justice has emerged. This entry examines a number of security measures and preventive measures, as well as new directions in cutting edge research and assesses where gaps remain in scholarship on security measures and preventive measures.