This course focuses on mathematical modeling of geochemical reactions, including aqueous phase and water-mineral reactions. We examine how the rates of reactions and fluid flow are interrelated and how to write numerical models that couple these processes. We start with reaction path modeling, and then move to reactive transport modeling. Relevant systems include 1D flow in porous media, 2D pore-network flow, and flow in fractures. Applications are drawn from a variety of problems relevant to environmental engineering, geosciences, and energy.