Cities are home to more than half of the human population worldwide. This share continues to grow, as cities remain critical centers of economic, cultural, and political life. While spatially they present an efficient solution to growing human populations, they give rise to their own sustainability challenges. Cities can suffer from social and economic inequities as well as disaster resilience, mobility, energy, water, air quality, waste, and biodiversity challenges. The AUA Acopian Center for the Environment brings together research, education and community outreach efforts and expertise on many of these challenges. It offers a systems level thinking about cities and their potential to offer sustainable solutions.