Historically, transient multiphase flow codes grew out of nuclear safety codes developed specifically to model nuclear plants during loss-of-coolant accidents (i.e., steam-water). These codes were heavily augmented by the oil and gas industry to deal with the more complex fluids, geometries, and thermodynamics associated with oil and gas production. Although several attempts were made to develop transient mixture models with a single momentum equation, development settled on two-fluid formulations, employing separate momentum equations for each phase (developed out of a force balance), as the best way forward. While such models were quite successful in simulating holdup and pressure drop during both steady-state and transient operations, many associated flow assurance issues (i.e., hydrates and hydrodynamic slugs) remain quite rudimentary. Also, the simulators themselves are terrifically slow by general computational fluid dynamics (CFD) standards. In the future, simple,, powerful mixture models may well make a comeback, supplanting the more complex two-fluid models. The rationale is 3-fold: First, the simplicity of these models will allow for much greater computational speed, resulting in the possibility to run much finer grids (on the order of a pipeline diameter) than currently used by transient pipeline simulators. Second, compositional tracking could be routinely used, allowing for much simpler simulations of, for example, complex networks of differing fluids. Third, flow assurance phenomena, such as hydrate formation, could be folded directly into the multiphase model on a fundamental level rather than as an ad hoc addition.