The Identity of Systems presents a structural model of why individuals, organizations, and societies resist change even when adaptation appears rational or necessary. Rather than treating identity as belief, culture, or ideology, this book defines identity as a system property: the set of constraints that preserve coherence across time. Using principles from systems theory, cybernetics, psychology, and organizational analysis, the book examines how boundaries, feedback loops, norms, and regulatory mechanisms stabilize behavior and suppress destabilizing variation. Change is not evaluated by benefit but by compatibility with identity-defining constraints. The framework explains why reform efforts often fail, why crises produce nonlinear transitions, and why stability mechanisms can increase long-term fragility. It also explores how systems reorganize after rupture, how new identities emerge, and how rigidification and fragmentation become common failure modes following disruption. Applications are examined across psychological, institutional, and societal systems, showing how the same regulatory dynamics govern personal defense mechanisms, organizational inertia, and political polarization. This book is written as a descriptive reference framework rather than a tactical guide. It provides structural explanation of persistence, resistance, and transformation in complex systems, offering a foundation for further work in psychology, organizational change, governance, and cognitive conflict.