Entrainment, E, in vertical gas-liquid annular Aows may be pictured as a balance between the rate of atomization, R(A), of the liquid layer and the rate of deposition of drops, R(D). Laboratory measurements of R(A), R(D) and E are reviewed. Theoretical analyses are discussed which picture R(A) as related to the growth of wavelets through a Kelvin-Helmholtz instability and R(D) 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 rate of the liquid in the film. Limitations of this approach, suggested by measurements of R(A) and R(D) at large liquid flows, are discussed.