Abstract:Beijing-Tianjin-Hebei region, Yangtze River Delta, and Guangdong-Hong Kong-Macao Greater Bay Area are the three largest urban agglomerations in China, and have been increasingly becoming the core engines of China's economic growth. Nevertheless, rapid regional urbanization accompanying high-intensity human activities has also brought enormous social and environmental pressure in these urban agglomerations, posing a grand challenge to urban and regional sustainability. This study conducts a cross-urban agglomeration comparison of the ecological and environmental changes over the past two decades. Using a combination of remotely sensed data, in-situ monitoring data, and environmental statistic data, the changes in four dimensions are quantified, namely ecological quality, environmental quality, resource and energy use efficiency, and eco-environmental governance capacity. Further, the effectiveness of environmental policies and initiatives is evaluated at the national and urban agglomeration levels. It is found that the quality of ecological land has increased steadily since 2000, with increased provision of ecosystem services. Air quality and water quality have been substantially improved, and the use of resources and energy efficiency have been greatly improved. The pollution emissions per unit GDP have been greatly reduced. The ecological and environmental infrastructures have become increasingly available, along with gradually strengthened governance capacity on environmental protection and ecological restoration. The National 10th to 13th Five-Year Plan for Environmental Protection played a significant role in environmental protection and ecological restoration across mega urban agglomerations. The environmental challenges, of urban agglomerations faced now and in the future, are also discussed, so as to the policies and actions that need to be addressed.