The chemical reactions that take place as water moves through the terrestrial and oceanic subsurface act as fundamental controls on many aspects of the Earth system, including global elemental cycling (C,N,O,S), critical zone development and function, the formation and alteration of chemical paleoclimate proxies, and contaminant fate and transport. As many of the environments and timescales involved are difficult to observe in-situ, reactive transport models (RTM?s) that simultaneously represent mass transport and chemical reactions have been developed to virtually explore these cryptic processes. This course is designed to introduce participants to the theory and applications of multi-component RTM?s across the Geosciences. Students will become familiar with underlying mathematical representations of reactive transport, numerical methods for solving RTM equations, along with the many ways in which these models are utilized in frontier research. Through this course, students will also gain hands-on experience building their own reactive transport models, becoming familiar with available multi-component reactive transport model software, and applying reactive transport modeling to their specific research interests.