Internal security is the act of keeping peace within the borders of a sovereign state or other self-governing territories, generally by upholding the national law and defending against internal security threats. Responsibility for internal security may range from police to paramilitary forces, and in exceptional circumstances, the military itself.

An internal security vehicle (ISV), also known as an armored security vehicle (ASV), is an armoured personnel carrier/armoured car used for supporting contingency operations.

The United States Senate's Special Subcommittee to Investigate the Administration of the Internal Security Act and Other Internal Security Laws, 1951–77, known more commonly as the Senate Internal Security Subcommittee (SISS) and sometimes the McCarran Committee, was authorized by S. 366, approved December 21, 1950, to study and investigate (1) the administration, operation, and enforcement of the Internal Security Act of 1950 (, also known as the McCarran Act) and other laws relating to espionage, sabotage, and the protection of the internal security of the United States and (2) the extent, nature, and effects of subversive activities in the United States "including, but not limited to, espionage, sabotage, and infiltration of persons who are or may be under the domination of the foreign government or organization controlling the world Communist movement or any movement seeking to overthrow the Government of the United States by force and violence".