Los Alamos National Laboratory’s Amanzi–ATS is a powerful and agile simulation tool for comprehensive environmental system analyses. Offering the most exhaustive suite of surface/subsurface processes, Amanzi–ATS is built on a flexible infrastructure that allows users to choose physical processes and their coupling without rewriting software. New physics can be incorporated into Amanzi–ATS in a fraction of the time it takes other high-performance frameworks. This level of runtime control gives users the power to customize simulations, combine processes and add new ones. Amanzi–ATS has been used to analyze pristine local watersheds in the Western United States, effects of wildfire damage on watersheds, subsurface contaminant transport at legacy waste sites and the effect of a warming climate on the Arctic tundra. Its comprehensive capabilities have yielded new insights into the behavior of environmental systems. Amanzi–ATS is fast becoming an invaluable tool for organizations such as the Department of Energy and the U.S. Geological Survey. As more physical processes are added, Amanzi–ATS stands to become the simulation software of choice for predictive science in environmental management.