This book is a practical guide to defending critical infrastructure in today’s cyber world. Critical infrastructure includes energy, power, water, transportation, manufacturing, and facility systems, along with the sensors and actuators that make up operational technology (OT). It presents a systems engineering framework for coordinating interdependent organizations and technologies, ensuring seamless performance across multiple fields. Bridging the traditional divide between information technology (IT), operational technology (OT), and cybersecurity, the book offers practical starting points and disciplined approaches for governance boards, project managers, and engineers. Clear guidance is provided on how to implement cybersecurity mitigations within critical infrastructure, making complex concepts accessible and actionable. Many professionals in infrastructure operations, engineering, and technical management feel unprepared to address cyber threats. This book fills that gap, serving as both a self-study resource and a refresher for those without formal training in computer science, cyber, or control systems—and as a teaching tool for explaining these principles to others. The goal is to present enduring cybersecurity concepts through proven systems engineering methods, along with new techniques to align stakeholders and resources. From technicians and operators to designers and decision-makers, readers will gain tools to connect cybersecurity practices with mission objectives. By combining systems engineering and cyber defense principles, this book equips professionals to work effectively with incident responders, analysts, engineers, and senior management. Decision-making skills are presented in a clear, easy-to-understand style, supported by candid questions, current strategies, and real-world, case-based examples.