This book is a summary of the work of dozens of innovators in the field of systems modeling and teaching for thirty years. Most of these innovators are members of or influenced by MIT's Systems Dynamics group, led by Jay Forrester, the group's founder. One may find dozens of books on "systems modeling" and "systems thinking," but that does not deny that there is a real need for a simple and inspiring book about systems, why we sometimes find them confusing, and how we can learn to manage them better and redesign them. That is, there is nothing better than this book in your hands, dear reader. Once we understand the relationship between structure and behavior, we can understand how systems work, what causes them to produce poor results, and discover how to direct them to better patterns of behavior. As our world continues to change rapidly and become more complex, systems thinking will help us see, manage and adapt to the wide range of options before us. It is a way of thinking that gives us the freedom to identify the root causes of problems and see new opportunities. So, what is the system? A system is a collection of things, people, cells, molecules, or whatever, that are interconnected in such a way that they produce their own pattern of behavior over time. A system may be collided, constrained, stimulated or directed by external forces, but the form of the system's response to these forces is its own, and this response is rarely simple in the real world. On the other hand, we have all dealt with complex systems, long before we learned logical analysis. We are complex systems; Our bodies are wonderful examples of integrated, interconnected, self-preserving complexity. Every person we meet, every institution, every animal, garden, tree, or forest is a complex system. We have formed a working understanding of how these systems work and how to work with them intuitively, without analysis, and often without words.