These attacks are designed to impede the system’s essential functions, such as causing a power blackout in a city block, which may have serious effects on the safety and security of people who rely on that system. Perimeter security measures, such as firewalls and access control systems, can deter or prevent cyber attacks originating from outside the system, but do not protect against insider attacks, which are typically initiated by agents familiar with the system. Much work has been done in designing network intrusion detection systems, but these systems tend to learn slowly and are generally helpless against unknown and adaptive threats. Vaccines, for example, are the body’s equivalent of trained intrusion detection systems. To create a cyber-physical immune system, it must, like the human body, become self-aware. To build a credible model of its own behaviour, the system must not just learn its digital behaviour, but also capture the behaviour of its physical subsystems.