This book offers a structured and practical approach to modern PLC development using object-oriented principles. It is a guide for engineers, programmers, and students seeking to harness the power of object-oriented programming (OOP) in the context of industrial automation with PLCs. The content focuses on the CODESYS development environment and Structured Text (ST), both of which support modern programming techniques while maintaining compatibility with real-time automation requirements. Through step-by-step demos and instructional examples, it demonstrates how modular, reusable code can enhance development efficiency, simplify ongoing maintenance, and enable scalable and flexible control system architectures. Key topics include: Structured Text fundamentals: conditions, loops, arrays, and functions Object-oriented concepts: classes, methods, and inheritance Advanced techniques: polymorphism, interfaces, and access control Modular design with reusable components and structured program flow Implementation of finite state machines and scalable application design Built around instructional demos and clear explanations, this book helps readers develop maintainable and modern control software in the CODESYS environment using proven programming techniques.