I have often referred to critical systems in this book, with only giving a high-level explanation of them. In Chap. 1 , I defined critical systems as those in the realms of defense, medical, aviation, and finance. There obviously are many more fields that can be added under the umbrella of critical systems, such as IT, cybersecurity, and the automotive industry, to name a few. It should also be noted that not all systems in these program areas are critical systems. How do we further define a critical system to understand why a system is considered critical? To do this, we must expand the definition and drill down into what criticality is.