Heavy oil makes up a major portion of the world's total oil resources. The high viscosity of heavy oil poses great challenges for its production and transportation through wells and pipelines. Experimental results show that the high-viscosity oil multiphase flow behaviors are very different from the low-viscosity oils. Significant discrepancies are displayed when the high-viscosity oil data are compared to the available mechanistic multiphase flow models, which were developed on the basis of low-viscosity oil experimental results. In this paper, the recent experimental findings on high-viscosity oil multiphase pipe flows are reviewed in contrast to low-viscosity oil multiphase flows. These include flow pattern, pressure gradient, holdups, slug characteristics, oil/water mixing, core-annular flow, gas-lift effect, and phase distributions. Individual closure relationships in the mechanistic models are discussed in light of experimental measurements and observations with high-viscosity oils. Future experimental and modeling needs are suggested.