Sustainability is no longer a feature. It is the operating system of the built environment. Real estate no longer operates in silos. Energy markets shape design decisions. Climate risk reshapes valuation. Policy defines performance thresholds. Technology governs daily operations. Capital responds in real time. What once lived in separate disciplines now functions as a single, interconnected system. This book explains that system. Sustainability as the Operating System offers a clear, practical framework for understanding how buildings, cities, infrastructure, and real estate markets actually work today. It moves beyond checklists, buzzwords, and certification chasing to show how environmental performance, financial outcomes, regulatory pressure, and operational intelligence have become inseparable. Written for practitioners, investors, and policymakers, this book connects the fundamentals of green building with the realities of modern development and ownership. It covers energy, water, materials, indoor environmental quality, transportation, site planning, artificial intelligence, and public policy not as standalone topics, but as parts of a single operating logic shaping every real estate decision. This is not a manifesto. It is not a trend report. It is a field guide for how the built environment is now designed, financed, governed, and managed. The professionals who succeed in the next decade will not be those who specialize narrowly. They will be the ones who understand how systems interact, how performance is measured, and how sustainability has become the baseline rather than the exception. This book is written for those ready to operate at that level.