We study multiphase and reactive fluid flow in the subsurface, emphasizing the emergence of preferential flow pathway due inherent heterogeneity in transport properties and alteration of these properties by mechanical and/or chemical deformation, e.g. fracturing or dissolution. We combine numerical simulations with experiments and theory to gain fundamental understanding of the underlying physics, which we apply towards environmental problems such as migration and recovery of water, gases and hydrocarbons, the associated subsurface contamination and geohazards, and soil wetting and drying.