Systems thinking is essential for approaching and creating sustainable solutions for real-world problems, as discussed in Part I of this book. The lack of systems thinking is evident almost everywhere where project costs and schedules are worse than planned or expected. Other common examples are where an anomaly in a small, seemingly uncritical part of the system causes a significant disaster and losses far beyond its cost or known significance. Chapter 3 gives easy-to-understand, convincing examples of real-world complex problems that require systems thinking. Unfortunately, our language, culture, and years of compartmentalized, disciplinary, and inside-the-box oriented education do not help! Encouraging systems thinking by pointing out the many examples of failures and fiascoes that could have been avoided is a good start but not enough. First, we must understand the details of clearly observable misconceptions or mental models that drive our linear compartmentalized thinking. These hamper our ability to find sustainable solutions to real-world problems. Then, we need reliable, practical, and actionable principles to guide our thinking, behavior, and methods for approaching various real-world problems. The idea is to apply principles that are always true to help maximize our understanding and improve the odds of success of our solutions. We call these Systems Thinking Principles or Systems Principles or SPs. We explain why we need the SPs in Section 5.1 followed by presenting each SP in detail in Sections 5.3 to 5.10. We recommend beginners to skip the rest of the chapter and move on to Chapters 6 and 7 that help develop an eye for the Systems Principles and how to apply them to understand real-world problems and find their best points of leverage.