Mass and volume balance are used to derive conservation equations for multiphase fluid flow in one dimension. These equations are solved in two limits. Firstly, ignoring the explicit effect of capillary pressure, the Buckley-Leverett solution is presented from which recovery plots as a function of pore volumes of water injected can be calculated. Secondly, capillary effects are included while viscous and buoyancy forces are ignored, to represent spontaneous imbibition in, for instance, a fractured reservoir. Example solutions for exemplar relative permeabilities and capillary pressures representing rocks of different wettability and a range of viscosity ratios are presented to illustrate the theory. The implications for field-scale displacement efficiency for different flooding processes are presented in detail, linking pore-scale dynamics and fluid configurations to large-scale recovery factor. The chapter ends with a discussion of the so-called “trillion barrel question” showing how the microscopic arrangement of fluids, especially the presence and flow of oil and water layers, presented in the previous chapters, can have a controlling impact on field-scale recovery.