Upgridding and Upscaling: What are they and why are they important? Over the last decade, the development of detailed three dimensional geologic descriptions has become common-place within asset and project teams worldwide. During this same period our industry has improved its ability to perform flow simulation on much larger models, but the geologic static models have continued to exceed our computational (flow simulation) capabilities. Upscaling describes the range of techniques used to develop the properties for these coarser, simpler models. Current best practice has moved away from “local” calculations, in which each portion of the reservoir is analyzed in isolation from each other, to “extended-local” and “global” techniques that take advantage of the adjacent portions of the detailed reservoir description to develop more robust effective properties. Upgridding describes the techniques that are used to design the spatial resolution of the coarser model, e.g., the simulation grid. The best upscaled models will retain detailed resolution where necessary and simplify the description where possible. Upgridding guides this variable resolution. Field examples from North America and the North Sea (tight gas and both high and low net-to-gross waterfloods) will be used to demonstrate the power of combining these approaches, giving rise to adaptive calculations that determine the best resolution and the best properties simultaneously. The examples will also demonstrate the powerful emerging trend of performing upscaling within the flow simulator itself, with the static geologic model as input and the spatial resolution for the simulation determined at run-time.