Annular flow is one of the most frequently observed flow patterns with gas-liquid two-phase flows in tubes or channels. In the annular flow pattern, a thin liquid film flows along the channel wall, while the gas flows in the center of the channel carrying liquid droplets in suspension. The fraction of the liquid flow rate that is transported as suspended droplets is quantified using the entrained liquid fraction (ELF), which is a key flow parameter in the analysis and modeling of annular flows. This review provides a critical assessment of ELF experimental data available in the open literature and of ELF prediction methods proposed to date. The experimental data assessment is carried out by means of a large ELF data bank collected from the literature (4175 data points from 53 literature studies; 10 fluids combinations; operating pressures from atmospheric to 20 MPa; experiments carried out with adiabatic, evaporating, and condensing flows through circular tubes, and non-circular channels with diameters from 3.02 to 155.7 mm), which is critically analyzed devoting special attention to important aspects not adequately addressed in previous studies, such as a cross-comparison between different ELF measuring techniques, and the analysis of flow development and gravity effects. The assessment of the ELF prediction methods focuses on 15 widely quoted methods, which are critically analyzed and whose prediction performance is evaluated against the measured data. The curated ELF experimental data bank is provided in full and usable form. Research gaps for further investigations are identified and discussed.