The concurrent flow of two phases through a pipe has been the object of increased study in recent years as a result of its recognized importance. It is well established that when a liquid phase and a gas phase flow simultaneously through a pipe, the resulting pressure drop is greater than if only a single phase were flowing. Under certain conditions, this "two-phase" pressure drop is further significantly increased when the flow is uphill. Knowledge of gas-liquid two-phase flow is therefore essential to solution of problems arising in such areas as natural gas transmission, oil production gathering systems, heat exchangers, and flow reactors.