Digital transformation has reshaped the industrial landscape. Control systems that operated in isolated environments for decades now coexist with IP networks, cloud services, remote access, and converged IT/OT architectures. This evolution has exponentially increased the attack surface and turned industrial infrastructures into strategic targets. Fundamentals of OT Cybersecurity provides a rigorous and structured perspective on the principles that must underpin the protection of industrial automation and control systems. Rather than simply transferring purely IT-based models into operational environments, the book examines the technical, organizational, and strategic particularities that distinguish industrial cybersecurity: the primacy of availability, the physical impact of incidents, the coexistence with legacy systems, and the complexity of industrial architectures. This book is not merely a technical introduction. It serves as a conceptual and strategic foundation for engineers, security leaders, industrial executives, and professionals who need to understand the terrain before designing solutions. Because in industrial environments, cybersecurity does not only protect information. It protects processes, critical infrastructure, and ultimately, the continuity of society.