SOLID principles are an essential part of object-oriented software development and have proven to be valuable tools for developing clean, maintainable and extensible code. In industrial automation engineering, especially in programming controllers with IEC 61131-3, it is of particular importance to develop robust and reliable systems. In this book, SOLID principles are presented in detail and explained with examples in IEC 61131-3. It also illustrates how the application of these principles improves the maintainability, extensibility, and reliability of software systems. In addition to the SOLID principles, the principles KISS, DRY, LoD and YAGNI are also presented. These do not belong to the group of SOLID principles, but they are a helpful addition.