Entrapment, E, in vertical gas-liquid annular flows may be pictured as a balance between the rate of atomization, RA, of the liquid layer and the rate of deposition of drops, R D. Laboratory measurements of RA, RD and E are reviewed. Theoretical analyses are discussed which picture RA as related to the growth of wavelets through a Kelvin-Helmholtz instability and RB as being directly proportional to the root-mean-square of the turbulent velocity fluctuations of the drops. An equation for E can be developed, which assumes that the deposition constant is independent of drop concentration and that the rate of atomization varies linearly with the flow rale of the liquid in the film. Limitations of this approach, suggested by measurements of RA and RD at large liquid flows, are discussed.