A clear and precise definition concept allows us to clearly identify the concepts with which we are dealing and allows us to conduct various reasoning in a more precise way. We will then embark on a journey of discovery and definition of key concepts to understand this book well. We will start with the concept of peace, engineering for peace building, general systems theory, what is a system, what is System’s Science, what are social systems, simple systems, complex systems, the emergent properties of these systems, adaptation skills and self-organization. complex social systems. peace building systems. The importance of dialogue. systems thinking and its key concepts. the importance of talking about systems, of applying systems thinking in practice, organizational cybernetics, complexity theory, and the value of systems thinking and complexity management in supporting decision makers in complex social systems. Finally, critical systemic thinking and the management of complexity.