MI5, or the Security Service, is the United Kingdom's domestic intelligence agency. Its primary role is to protect the country from threats such as terrorism, espionage, and cyber-attacks. Established in 1909, MI5 works closely with other intelligence agencies, including MI6 and GCHQ, to ensure national security. The agency gathers intelligence, conducts investigations, and collaborates with law enforcement to prevent criminal activities. MI5 also focuses on counter-terrorism efforts, monitoring potential threats, and providing guidance to the government on security matters. Its headquarters is located in London, and it operates under the authority of the Home Office.