We study the displacement flow of two Newtonian fluids in an inclined pipe. The fluids have the same viscosity but different densities. The displacing fluid is denser than the displaced fluid and is placed above the displaced fluid (i.e. a density-unstable configuration). Three dimensionless groups describe these flows: a densimetric Froude numberFr, a Reynolds number Re and the pipe inclination. A rich variety of flow phenomena can be observed in varying these parameters. At pipe inclinations close to horizontal and for moderate Re/Fr, the flows are laminar and viscous-dominated. A simple thin-film style modelling approach proves very effective in understanding these flows. As the inclination angles are reduced and the pipe becomes progressively vertical, we transition through buoyancy-dominated inertial exchange regimes to mixed and fully diffusive regimes. We give a leading order characterization of these flows. Joint work with K. Alba and S.M. Taghavi.