Industrial control systems are an integral part of critical infrastructure, helping facilitate operations in vital sectors such as electricity, oil and gas, water, transportation, and chemical. A growing issue with cybersecurity and its impact on industrial control systems have highlighted some fundamental risks to critical infrastructures. To address cybersecurity issues for industrial control systems, a clear understanding of the security challenges and specific defensive countermeasures is required. A holistic approach, one that uses specific countermeasures to create an aggregated security posture, can help defend against cybersecurity threats and vulnerabilities that affect an industrial control system. This approach, often referred to as "defense-in-depth," can be applied to industrial control systems and can provide for a flexible and useable framework for improving cybersecurity defenses. Concerns in regard to cybersecurity and control systems are related to both the legacy nature of some of the systems as well as the growing trend to connect industrial control systems to other networks. These concerns have lead to a number of identified vulnerabilities and have introduced new categories of threats that have not been seen before in the industrial control systems domain. Many of the legacy systems may not have appropriate security capabilities that can defend against modern day threats, and the requirements for availability can preclude using contemporary cybersecurity solutions. An industrial control system's connectivity to a corporate, vendor, or peer network can exacerbate this problem. This book provides insight into some of the more prominent cyber risk issues and presents them in the context of industrial control systems. It provides commentary on how mitigations strategies can be developed for specific problems and provides direction on how to create a defense-in-depth security program for control system environments. The goal is to provide guidance regarding cyber mitigation strategies and how to apply them specifically to an industrial control systems environment.