About 100 years ago, long before the invention of the computer, Lewis Fry Richardson developed the first CFD code. A lot has changed since then. Today, CFD is used intensively for a wide variety of applications, not the least of those involving sustainability. Sustainability can be concisely defined as “meeting the needs of the present without compromising the future”. In this context, one of the most important and challenging arenas is the built environment. This lecture demonstrates the key role of CFD towards sustainability of buildings and cities by a number of large case studies, from the improvement of air quality in buildings and urban areas to reducing heat waves over ensuring safe navigation of container and cruise ships in ports to tackling respiratory pandemics in sports stadiums. The lecture will show how each of these studies would not have been possible without the power of high-quality CFD simulations