The simultaneous flow of multiple fluids through permeable rocks and sediments is vital to global challenges such as CO2 sequestration, geological energy storage and subsurface contaminant transport. Large-scale, continuum models of multiphase flow are semi-empirical, frequently leading to poor predictions. Fundamental pore-scale studies, on the other hand, are commonly performed below the representative elementary volume (REV). This proposal aims to fill the gap between the averaged description of multiphase flow (cm-scale core samples) and the fluid flow physics at the pore scale (~µm). There are three key unknowns: 1) how does the REV for multiphase flow depend on the flow conditions; 2) what is the influence of pore-scale dynamics on cm-scale fluid distributions in rocks; and 3) what is the effect of mm-to-cm heterogeneity on pore-scale behavior? This will be addressed by investigating the pore-scale fluid distributions in sample sizes an order of magnitude larger than typical studies, using repeated experiments and samples with different types and levels of heterogeneity. The latest advances in multi-scale µCT imaging and image analysis will be combined with specially designed flow experiments. This project will be the first systematic study to experimentally characterize multiphase flow from the pore to the core scale. Bridging this gap is a crucial step in achieving better models of geological carbon sequestration, renewable energy (storage) and groundwater resources.